1 The LTTng Documentation
2 =======================
3 Philippe Proulx <pproulx@efficios.com>
4 v2.10, 12 November 2018
7 include::../common/copyright.txt[]
10 include::../common/welcome.txt[]
13 include::../common/audience.txt[]
17 === What's in this documentation?
19 The LTTng Documentation is divided into the following sections:
21 * **<<nuts-and-bolts,Nuts and bolts>>** explains the
22 rudiments of software tracing and the rationale behind the
25 You can skip this section if you’re familiar with software tracing and
26 with the LTTng project.
28 * **<<installing-lttng,Installation>>** describes the steps to
29 install the LTTng packages on common Linux distributions and from
32 You can skip this section if you already properly installed LTTng on
35 * **<<getting-started,Quick start>>** is a concise guide to
36 getting started quickly with LTTng kernel and user space tracing.
38 We recommend this section if you're new to LTTng or to software tracing
41 You can skip this section if you're not new to LTTng.
43 * **<<core-concepts,Core concepts>>** explains the concepts at
46 It's a good idea to become familiar with the core concepts
47 before attempting to use the toolkit.
49 * **<<plumbing,Components of LTTng>>** describes the various components
50 of the LTTng machinery, like the daemons, the libraries, and the
51 command-line interface.
52 * **<<instrumenting,Instrumentation>>** shows different ways to
53 instrument user applications and the Linux kernel.
55 Instrumenting source code is essential to provide a meaningful
58 You can skip this section if you do not have a programming background.
60 * **<<controlling-tracing,Tracing control>>** is divided into topics
61 which demonstrate how to use the vast array of features that
62 LTTng{nbsp}{revision} offers.
63 * **<<reference,Reference>>** contains reference tables.
64 * **<<glossary,Glossary>>** is a specialized dictionary of terms related
65 to LTTng or to the field of software tracing.
68 include::../common/convention.txt[]
71 include::../common/acknowledgements.txt[]
75 == What's new in LTTng {revision}?
77 LTTng{nbsp}{revision} bears the name _KeKriek_. From
78 http://brasseriedunham.com/[Brasserie Dunham], the _**KeKriek**_ is a
79 sour mashed golden wheat ale fermented with local sour cherries from
80 Tougas orchards. Fresh sweet cherry notes with some tartness, lively
81 carbonation with a dry finish.
83 New features and changes in LTTng{nbsp}{revision}:
85 * **Tracing control**:
86 ** You can put more than one wildcard special character (`*`), and not
87 only at the end, when you <<enabling-disabling-events,create an event
88 rule>>, in both the instrumentation point name and the literal
90 link:/man/1/lttng-enable-event/v{revision}/#doc-filter-syntax[filter expressions]:
95 # lttng enable-event --kernel 'x86_*_local_timer_*' \
96 --filter='name == "*a*b*c*d*e" && count >= 23'
103 $ lttng enable-event --userspace '*_my_org:*msg*'
107 ** New trigger and notification API for
108 <<liblttng-ctl-lttng,`liblttng-ctl`>>. This new subsystem allows you
109 to register triggers which emit a notification when a given
110 condition is satisfied. As of LTTng{nbsp}{revision}, only
111 <<channel,channel>> buffer usage conditions are available.
112 Documentation is available in the
113 https://github.com/lttng/lttng-tools/tree/stable-{revision}/include/lttng[`liblttng-ctl`
115 <<notif-trigger-api,Get notified when a channel's buffer usage is too
118 ** You can now embed the whole textual LTTng-tools man pages into the
119 executables at build time with the `--enable-embedded-help`
120 configuration option. Thanks to this option, you don't need the
121 http://www.methods.co.nz/asciidoc/[AsciiDoc] and
122 https://directory.fsf.org/wiki/Xmlto[xmlto] tools at build time, and
123 a manual pager at run time, to get access to this documentation.
125 * **User space tracing**:
126 ** New blocking mode: an LTTng-UST tracepoint can now block until
127 <<channel,sub-buffer>> space is available instead of discarding event
128 records in <<channel-overwrite-mode-vs-discard-mode,discard mode>>.
129 With this feature, you can be sure that no event records are
130 discarded during your application's execution at the expense of
133 For example, the following command lines create a user space tracing
134 channel with an infinite blocking timeout and run an application
135 instrumented with LTTng-UST which is explicitly allowed to block:
141 $ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
142 $ lttng enable-event --userspace --channel=blocking-channel --all
144 $ LTTNG_UST_ALLOW_BLOCKING=1 my-app
148 See the complete <<blocking-timeout-example,blocking timeout example>>.
150 * **Linux kernel tracing**:
151 ** Linux 4.10, 4.11, and 4.12 support.
152 ** The thread state dump events recorded by LTTng-modules now contain
153 the task's CPU identifier. This improves the precision of the
154 scheduler model for analyses.
155 ** Extended man:socketpair(2) system call tracing data.
161 What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
162 generation_ is a modern toolkit for tracing Linux systems and
163 applications. So your first question might be:
170 As the history of software engineering progressed and led to what
171 we now take for granted--complex, numerous and
172 interdependent software applications running in parallel on
173 sophisticated operating systems like Linux--the authors of such
174 components, software developers, began feeling a natural
175 urge to have tools that would ensure the robustness and good performance
176 of their masterpieces.
178 One major achievement in this field is, inarguably, the
179 https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
180 an essential tool for developers to find and fix bugs. But even the best
181 debugger won't help make your software run faster, and nowadays, faster
182 software means either more work done by the same hardware, or cheaper
183 hardware for the same work.
185 A _profiler_ is often the tool of choice to identify performance
186 bottlenecks. Profiling is suitable to identify _where_ performance is
187 lost in a given software. The profiler outputs a profile, a statistical
188 summary of observed events, which you may use to discover which
189 functions took the most time to execute. However, a profiler won't
190 report _why_ some identified functions are the bottleneck. Bottlenecks
191 might only occur when specific conditions are met, conditions that are
192 sometimes impossible to capture by a statistical profiler, or impossible
193 to reproduce with an application altered by the overhead of an
194 event-based profiler. For a thorough investigation of software
195 performance issues, a history of execution is essential, with the
196 recorded values of variables and context fields you choose, and
197 with as little influence as possible on the instrumented software. This
198 is where tracing comes in handy.
200 _Tracing_ is a technique used to understand what goes on in a running
201 software system. The software used for tracing is called a _tracer_,
202 which is conceptually similar to a tape recorder. When recording,
203 specific instrumentation points placed in the software source code
204 generate events that are saved on a giant tape: a _trace_ file. You
205 can trace user applications and the operating system at the same time,
206 opening the possibility of resolving a wide range of problems that would
207 otherwise be extremely challenging.
209 Tracing is often compared to _logging_. However, tracers and loggers are
210 two different tools, serving two different purposes. Tracers are
211 designed to record much lower-level events that occur much more
212 frequently than log messages, often in the range of thousands per
213 second, with very little execution overhead. Logging is more appropriate
214 for a very high-level analysis of less frequent events: user accesses,
215 exceptional conditions (errors and warnings, for example), database
216 transactions, instant messaging communications, and such. Simply put,
217 logging is one of the many use cases that can be satisfied with tracing.
219 The list of recorded events inside a trace file can be read manually
220 like a log file for the maximum level of detail, but it is generally
221 much more interesting to perform application-specific analyses to
222 produce reduced statistics and graphs that are useful to resolve a
223 given problem. Trace viewers and analyzers are specialized tools
226 In the end, this is what LTTng is: a powerful, open source set of
227 tools to trace the Linux kernel and user applications at the same time.
228 LTTng is composed of several components actively maintained and
229 developed by its link:/community/#where[community].
232 [[lttng-alternatives]]
233 === Alternatives to noch:{LTTng}
235 Excluding proprietary solutions, a few competing software tracers
238 * https://github.com/dtrace4linux/linux[dtrace4linux] is a port of
239 Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets
240 user scripts and is responsible for loading code into the
241 Linux kernel for further execution and collecting the outputted data.
242 * https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a
243 subsystem in the Linux kernel in which a virtual machine can execute
244 programs passed from the user space to the kernel. You can attach
245 such programs to tracepoints and KProbes thanks to a system call, and
246 they can output data to the user space when executed thanks to
247 different mechanisms (pipe, VM register values, and eBPF maps, to name
249 * https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]
250 is the de facto function tracer of the Linux kernel. Its user
251 interface is a set of special files in sysfs.
252 * https://perf.wiki.kernel.org/[perf] is
253 a performance analyzing tool for Linux which supports hardware
254 performance counters, tracepoints, as well as other counters and
255 types of probes. perf's controlling utility is the cmd:perf command
257 * http://linux.die.net/man/1/strace[strace]
258 is a command-line utility which records system calls made by a
259 user process, as well as signal deliveries and changes of process
260 state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace]
261 to fulfill its function.
262 * http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
263 analyze Linux kernel events. You write scripts, or _chisels_ in
264 sysdig's jargon, in Lua and sysdig executes them while the system is
265 being traced or afterwards. sysdig's interface is the cmd:sysdig
266 command-line tool as well as the curses-based cmd:csysdig tool.
267 * https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and
268 user space tracer which uses custom user scripts to produce plain text
269 traces. SystemTap converts the scripts to the C language, and then
270 compiles them as Linux kernel modules which are loaded to produce
271 trace data. SystemTap's primary user interface is the cmd:stap
274 The main distinctive features of LTTng is that it produces correlated
275 kernel and user space traces, as well as doing so with the lowest
276 overhead amongst other solutions. It produces trace files in the
277 http://diamon.org/ctf[CTF] format, a file format optimized
278 for the production and analyses of multi-gigabyte data.
280 LTTng is the result of more than 10 years of active open source
281 development by a community of passionate developers.
282 LTTng{nbsp}{revision} is currently available on major desktop and server
285 The main interface for tracing control is a single command-line tool
286 named cmd:lttng. The latter can create several tracing sessions, enable
287 and disable events on the fly, filter events efficiently with custom
288 user expressions, start and stop tracing, and much more. LTTng can
289 record the traces on the file system or send them over the network, and
290 keep them totally or partially. You can view the traces once tracing
291 becomes inactive or in real-time.
293 <<installing-lttng,Install LTTng now>> and
294 <<getting-started,start tracing>>!
300 **LTTng** is a set of software <<plumbing,components>> which interact to
301 <<instrumenting,instrument>> the Linux kernel and user applications, and
302 to <<controlling-tracing,control tracing>> (start and stop
303 tracing, enable and disable event rules, and the rest). Those
304 components are bundled into the following packages:
306 * **LTTng-tools**: Libraries and command-line interface to
308 * **LTTng-modules**: Linux kernel modules to instrument and
310 * **LTTng-UST**: Libraries and Java/Python packages to instrument and
311 trace user applications.
313 Most distributions mark the LTTng-modules and LTTng-UST packages as
314 optional when installing LTTng-tools (which is always required). In the
315 following sections, we always provide the steps to install all three,
318 * You only need to install LTTng-modules if you intend to trace the
320 * You only need to install LTTng-UST if you intend to trace user
324 .Availability of LTTng{nbsp}{revision} for major Linux distributions as of 12 November 2018.
326 |Distribution |Available in releases |Alternatives
328 |https://www.ubuntu.com/[Ubuntu]
329 |<<ubuntu,Ubuntu{nbsp}18.04 _Bionic Beaver_>>.
331 Ubuntu{nbsp}14.04 _Trusty Tahr_, Ubuntu{nbsp}16.04 _Xenial Xerus_,
332 and Ubuntu{nbsp}18.04 _Bionic Beaver_:
333 <<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
334 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
336 |https://getfedora.org/[Fedora]
337 |<<fedora,"Fedora{nbsp}27, Fedora{nbsp}28, and Fedora{nbsp}29">>.
338 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
340 |https://www.debian.org/[Debian]
341 |<<debian,Debian "buster" (testing)>>.
342 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
344 |https://www.archlinux.org/[Arch Linux]
345 |<<arch-linux,Current Arch Linux build>>.
346 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
348 |https://alpinelinux.org/[Alpine Linux]
349 |<<alpine-linux,Alpine Linux{nbsp}3.7 and Alpine Linux{nbsp}3.8>>.
350 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
352 |https://www.opensuse.org/[openSUSE]
353 |<<opensuse,openSUSE Leap{nbsp}15.0>>.
354 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
356 |https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
357 |See http://packages.efficios.com/[EfficiOS Enterprise Packages].
360 |https://buildroot.org/[Buildroot]
361 |<<buildroot,"Buildroot{nbsp}2018.02, Buildroot{nbsp}2018.05,
362 Buildroot{nbsp}2018.08, and Buildroot{nbsp}2018.11">>.
363 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
365 |http://www.openembedded.org/wiki/Main_Page[OpenEmbedded] and
366 https://www.yoctoproject.org/[Yocto]
368 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
373 === [[ubuntu-official-repositories]]Ubuntu
375 LTTng{nbsp}{revision} is available on Ubuntu{nbsp}18.04 _Bionic Beaver_.
376 For previous supported releases of Ubuntu,
377 <<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
379 To install LTTng{nbsp}{revision} on Ubuntu{nbsp}18.04 _Bionic Beaver_:
381 . Install the main LTTng{nbsp}{revision} packages:
386 # apt-get install lttng-tools
387 # apt-get install lttng-modules-dkms
388 # apt-get install liblttng-ust-dev
392 . **If you need to instrument and trace
393 <<java-application,Java applications>>**, install the LTTng-UST
399 # apt-get install liblttng-ust-agent-java
403 . **If you need to instrument and trace
404 <<python-application,Python{nbsp}3 applications>>**, install the
405 LTTng-UST Python agent:
410 # apt-get install python3-lttngust
416 ==== noch:{LTTng} Stable {revision} PPA
418 The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng
419 Stable{nbsp}{revision} PPA] offers the latest stable
420 LTTng{nbsp}{revision} packages for:
422 * Ubuntu{nbsp}14.04 _Trusty Tahr_
423 * Ubuntu{nbsp}16.04 _Xenial Xerus_
424 * Ubuntu{nbsp}18.04 _Bionic Beaver_
426 To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA:
428 . Add the LTTng Stable{nbsp}{revision} PPA repository and update the
434 # apt-add-repository ppa:lttng/stable-2.10
439 . Install the main LTTng{nbsp}{revision} packages:
444 # apt-get install lttng-tools
445 # apt-get install lttng-modules-dkms
446 # apt-get install liblttng-ust-dev
450 . **If you need to instrument and trace
451 <<java-application,Java applications>>**, install the LTTng-UST
457 # apt-get install liblttng-ust-agent-java
461 . **If you need to instrument and trace
462 <<python-application,Python{nbsp}3 applications>>**, install the
463 LTTng-UST Python agent:
468 # apt-get install python3-lttngust
476 To install LTTng{nbsp}{revision} on Fedora{nbsp}27, Fedora{nbsp}28,
479 . Install the LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision}
485 # yum install lttng-tools
486 # yum install lttng-ust
490 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
496 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.10.tar.bz2 &&
497 tar -xf lttng-modules-latest-2.10.tar.bz2 &&
498 cd lttng-modules-2.10.* &&
500 sudo make modules_install &&
506 .Java and Python application instrumentation and tracing
508 If you need to instrument and trace <<java-application,Java
509 applications>> on Fedora, you need to build and install
510 LTTng-UST{nbsp}{revision} <<building-from-source,from source>> and pass
511 the `--enable-java-agent-jul`, `--enable-java-agent-log4j`, or
512 `--enable-java-agent-all` options to the `configure` script, depending
513 on which Java logging framework you use.
515 If you need to instrument and trace <<python-application,Python
516 applications>> on Fedora, you need to build and install
517 LTTng-UST{nbsp}{revision} from source and pass the
518 `--enable-python-agent` option to the `configure` script.
525 To install LTTng{nbsp}{revision} on Debian "buster" (testing):
527 . Install the main LTTng{nbsp}{revision} packages:
532 # apt-get install lttng-modules-dkms
533 # apt-get install liblttng-ust-dev
534 # apt-get install lttng-tools
538 . **If you need to instrument and trace <<java-application,Java
539 applications>>**, install the LTTng-UST Java agent:
544 # apt-get install liblttng-ust-agent-java
548 . **If you need to instrument and trace <<python-application,Python
549 applications>>**, install the LTTng-UST Python agent:
554 # apt-get install python3-lttngust
562 LTTng-UST{nbsp}{revision} is available in Arch Linux's _Community_
563 repository, while LTTng-tools{nbsp}{revision} and
564 LTTng-modules{nbsp}{revision} are available in the
565 https://aur.archlinux.org/[AUR].
567 To install LTTng{nbsp}{revision} on Arch Linux, using
568 https://github.com/rmarquis/pacaur[pacaur] for the AUR packages:
570 . Install the main LTTng{nbsp}{revision} packages:
575 # pacman -Sy lttng-ust
576 $ pikaur -Sy lttng-tools
577 $ pikaur -Sy lttng-modules
581 . **If you need to instrument and trace <<python-application,Python
582 applications>>**, install the LTTng-UST Python agent:
587 # pacman -Sy python-lttngust
588 # pacman -Sy python2-lttngust
596 To install LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision} on
597 Alpine Linux{nbsp}3.7 or Alpine Linux{nbsp}3.8:
599 . Add the LTTng packages:
604 # apk add lttng-tools
605 # apk add lttng-ust-dev
609 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
615 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.10.tar.bz2 &&
616 tar -xf lttng-modules-latest-2.10.tar.bz2 &&
617 cd lttng-modules-2.10.* &&
619 sudo make modules_install &&
628 To install LTTng{nbsp}{revision} on openSUSE Leap{nbsp}15.0:
630 * Install the main LTTng{nbsp}{revision} packages:
635 sudo zypper install lttng-tools
636 sudo zypper install lttng-modules
637 sudo zypper install lttng-ust-devel
642 .Java and Python application instrumentation and tracing
644 If you need to instrument and trace <<java-application,Java
645 applications>> on openSUSE, you need to build and install
646 LTTng-UST{nbsp}{revision} <<building-from-source,from source>> and pass
647 the `--enable-java-agent-jul`, `--enable-java-agent-log4j`, or
648 `--enable-java-agent-all` options to the `configure` script, depending
649 on which Java logging framework you use.
651 If you need to instrument and trace <<python-application,Python
652 applications>> on openSUSE, you need to build and install
653 LTTng-UST{nbsp}{revision} from source and pass the
654 `--enable-python-agent` option to the `configure` script.
658 [[enterprise-distributions]]
659 === RHEL, SUSE, and other enterprise distributions
661 To install LTTng on enterprise Linux distributions, such as Red Hat
662 Enterprise Linux (RHEL) and SUSE Linux Enterprise Server (SUSE), please
663 see http://packages.efficios.com/[EfficiOS Enterprise Packages].
666 [[building-from-source]]
667 === Build from source
669 To build and install LTTng{nbsp}{revision} from source:
671 . Using your distribution's package manager, or from source, install
672 the following dependencies of LTTng-tools and LTTng-UST:
675 * https://sourceforge.net/projects/libuuid/[libuuid]
676 * http://directory.fsf.org/wiki/Popt[popt]
677 * http://liburcu.org/[Userspace RCU]
678 * http://www.xmlsoft.org/[libxml2]
681 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
687 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.10.tar.bz2 &&
688 tar -xf lttng-modules-latest-2.10.tar.bz2 &&
689 cd lttng-modules-2.10.* &&
691 sudo make modules_install &&
696 . Download, build, and install the latest LTTng-UST{nbsp}{revision}:
702 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.10.tar.bz2 &&
703 tar -xf lttng-ust-latest-2.10.tar.bz2 &&
704 cd lttng-ust-2.10.* &&
714 .Java and Python application tracing
716 If you need to instrument and trace <<java-application,Java
717 applications>>, pass the `--enable-java-agent-jul`,
718 `--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
719 `configure` script, depending on which Java logging framework you use.
721 If you need to instrument and trace <<python-application,Python
722 applications>>, pass the `--enable-python-agent` option to the
723 `configure` script. You can set the `PYTHON` environment variable to the
724 path to the Python interpreter for which to install the LTTng-UST Python
732 By default, LTTng-UST libraries are installed to
733 dir:{/usr/local/lib}, which is the de facto directory in which to
734 keep self-compiled and third-party libraries.
736 When <<building-tracepoint-providers-and-user-application,linking an
737 instrumented user application with `liblttng-ust`>>:
739 * Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
741 * Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
742 man:gcc(1), man:g++(1), or man:clang(1).
746 . Download, build, and install the latest LTTng-tools{nbsp}{revision}:
752 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.10.tar.bz2 &&
753 tar -xf lttng-tools-latest-2.10.tar.bz2 &&
754 cd lttng-tools-2.10.* &&
762 TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
763 previous steps automatically for a given version of LTTng and confine
764 the installed files in a specific directory. This can be useful to test
765 LTTng without installing it on your system.
771 This is a short guide to get started quickly with LTTng kernel and user
774 Before you follow this guide, make sure to <<installing-lttng,install>>
777 This tutorial walks you through the steps to:
779 . <<tracing-the-linux-kernel,Trace the Linux kernel>>.
780 . <<tracing-your-own-user-application,Trace a user application>> written
782 . <<viewing-and-analyzing-your-traces,View and analyze the
786 [[tracing-the-linux-kernel]]
787 === Trace the Linux kernel
789 The following command lines start with the `#` prompt because you need
790 root privileges to trace the Linux kernel. You can also trace the kernel
791 as a regular user if your Unix user is a member of the
792 <<tracing-group,tracing group>>.
794 . Create a <<tracing-session,tracing session>> which writes its traces
795 to dir:{/tmp/my-kernel-trace}:
800 # lttng create my-kernel-session --output=/tmp/my-kernel-trace
804 . List the available kernel tracepoints and system calls:
809 # lttng list --kernel
810 # lttng list --kernel --syscall
814 . Create <<event,event rules>> which match the desired instrumentation
815 point names, for example the `sched_switch` and `sched_process_fork`
816 tracepoints, and the man:open(2) and man:close(2) system calls:
821 # lttng enable-event --kernel sched_switch,sched_process_fork
822 # lttng enable-event --kernel --syscall open,close
826 You can also create an event rule which matches _all_ the Linux kernel
827 tracepoints (this will generate a lot of data when tracing):
832 # lttng enable-event --kernel --all
836 . <<basic-tracing-session-control,Start tracing>>:
845 . Do some operation on your system for a few seconds. For example,
846 load a website, or list the files of a directory.
847 . <<basic-tracing-session-control,Stop tracing>> and destroy the
858 The man:lttng-destroy(1) command does not destroy the trace data; it
859 only destroys the state of the tracing session.
861 . For the sake of this example, make the recorded trace accessible to
867 # chown -R $(whoami) /tmp/my-kernel-trace
871 See <<viewing-and-analyzing-your-traces,View and analyze the
872 recorded events>> to view the recorded events.
875 [[tracing-your-own-user-application]]
876 === Trace a user application
878 This section steps you through a simple example to trace a
879 _Hello world_ program written in C.
881 To create the traceable user application:
883 . Create the tracepoint provider header file, which defines the
884 tracepoints and the events they can generate:
890 #undef TRACEPOINT_PROVIDER
891 #define TRACEPOINT_PROVIDER hello_world
893 #undef TRACEPOINT_INCLUDE
894 #define TRACEPOINT_INCLUDE "./hello-tp.h"
896 #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
899 #include <lttng/tracepoint.h>
909 ctf_string(my_string_field, my_string_arg)
910 ctf_integer(int, my_integer_field, my_integer_arg)
914 #endif /* _HELLO_TP_H */
916 #include <lttng/tracepoint-event.h>
920 . Create the tracepoint provider package source file:
926 #define TRACEPOINT_CREATE_PROBES
927 #define TRACEPOINT_DEFINE
929 #include "hello-tp.h"
933 . Build the tracepoint provider package:
938 $ gcc -c -I. hello-tp.c
942 . Create the _Hello World_ application source file:
949 #include "hello-tp.h"
951 int main(int argc, char *argv[])
955 puts("Hello, World!\nPress Enter to continue...");
958 * The following getchar() call is only placed here for the purpose
959 * of this demonstration, to pause the application in order for
960 * you to have time to list its tracepoints. It is not
966 * A tracepoint() call.
968 * Arguments, as defined in hello-tp.h:
970 * 1. Tracepoint provider name (required)
971 * 2. Tracepoint name (required)
972 * 3. my_integer_arg (first user-defined argument)
973 * 4. my_string_arg (second user-defined argument)
975 * Notice the tracepoint provider and tracepoint names are
976 * NOT strings: they are in fact parts of variables that the
977 * macros in hello-tp.h create.
979 tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
981 for (x = 0; x < argc; ++x) {
982 tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
985 puts("Quitting now!");
986 tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
993 . Build the application:
1002 . Link the application with the tracepoint provider package,
1003 `liblttng-ust`, and `libdl`:
1008 $ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
1012 Here's the whole build process:
1015 .User space tracing tutorial's build steps.
1016 image::ust-flow.png[]
1018 To trace the user application:
1020 . Run the application with a few arguments:
1025 $ ./hello world and beyond
1034 Press Enter to continue...
1038 . Start an LTTng <<lttng-sessiond,session daemon>>:
1043 $ lttng-sessiond --daemonize
1047 Note that a session daemon might already be running, for example as
1048 a service that the distribution's service manager started.
1050 . List the available user space tracepoints:
1055 $ lttng list --userspace
1059 You see the `hello_world:my_first_tracepoint` tracepoint listed
1060 under the `./hello` process.
1062 . Create a <<tracing-session,tracing session>>:
1067 $ lttng create my-user-space-session
1071 . Create an <<event,event rule>> which matches the
1072 `hello_world:my_first_tracepoint` event name:
1077 $ lttng enable-event --userspace hello_world:my_first_tracepoint
1081 . <<basic-tracing-session-control,Start tracing>>:
1090 . Go back to the running `hello` application and press Enter. The
1091 program executes all `tracepoint()` instrumentation points and exits.
1092 . <<basic-tracing-session-control,Stop tracing>> and destroy the
1103 The man:lttng-destroy(1) command does not destroy the trace data; it
1104 only destroys the state of the tracing session.
1106 By default, LTTng saves the traces in
1107 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
1108 where +__name__+ is the tracing session name. The
1109 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
1111 See <<viewing-and-analyzing-your-traces,View and analyze the
1112 recorded events>> to view the recorded events.
1115 [[viewing-and-analyzing-your-traces]]
1116 === View and analyze the recorded events
1118 Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
1119 kernel>> and <<tracing-your-own-user-application,Trace a user
1120 application>> tutorials, you can inspect the recorded events.
1122 Many tools are available to read LTTng traces:
1124 * **cmd:babeltrace** is a command-line utility which converts trace
1125 formats; it supports the format that LTTng produces, CTF, as well as a
1126 basic text output which can be ++grep++ed. The cmd:babeltrace command
1127 is part of the http://diamon.org/babeltrace[Babeltrace] project.
1128 * Babeltrace also includes
1129 **https://www.python.org/[Python] bindings** so
1130 that you can easily open and read an LTTng trace with your own script,
1131 benefiting from the power of Python.
1132 * http://tracecompass.org/[**Trace Compass**]
1133 is a graphical user interface for viewing and analyzing any type of
1134 logs or traces, including LTTng's.
1135 * https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
1136 project which includes many high-level analyses of LTTng kernel
1137 traces, like scheduling statistics, interrupt frequency distribution,
1138 top CPU usage, and more.
1140 NOTE: This section assumes that the traces recorded during the previous
1141 tutorials were saved to their default location, in the
1142 dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
1143 environment variable defaults to `$HOME` if not set.
1146 [[viewing-and-analyzing-your-traces-bt]]
1147 ==== Use the cmd:babeltrace command-line tool
1149 The simplest way to list all the recorded events of a trace is to pass
1150 its path to cmd:babeltrace with no options:
1154 $ babeltrace ~/lttng-traces/my-user-space-session*
1157 cmd:babeltrace finds all traces recursively within the given path and
1158 prints all their events, merging them in chronological order.
1160 You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
1165 $ babeltrace /tmp/my-kernel-trace | grep _switch
1168 You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
1169 count the recorded events:
1173 $ babeltrace /tmp/my-kernel-trace | grep _open | wc --lines
1177 [[viewing-and-analyzing-your-traces-bt-python]]
1178 ==== Use the Babeltrace Python bindings
1180 The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
1181 is useful to isolate events by simple matching using man:grep(1) and
1182 similar utilities. However, more elaborate filters, such as keeping only
1183 event records with a field value falling within a specific range, are
1184 not trivial to write using a shell. Moreover, reductions and even the
1185 most basic computations involving multiple event records are virtually
1186 impossible to implement.
1188 Fortunately, Babeltrace ships with Python 3 bindings which makes it easy
1189 to read the event records of an LTTng trace sequentially and compute the
1190 desired information.
1192 The following script accepts an LTTng Linux kernel trace path as its
1193 first argument and prints the short names of the top 5 running processes
1194 on CPU 0 during the whole trace:
1199 from collections import Counter
1205 if len(sys.argv) != 2:
1206 msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
1207 print(msg, file=sys.stderr)
1210 # A trace collection contains one or more traces
1211 col = babeltrace.TraceCollection()
1213 # Add the trace provided by the user (LTTng traces always have
1215 if col.add_trace(sys.argv[1], 'ctf') is None:
1216 raise RuntimeError('Cannot add trace')
1218 # This counter dict contains execution times:
1220 # task command name -> total execution time (ns)
1221 exec_times = Counter()
1223 # This contains the last `sched_switch` timestamp
1227 for event in col.events:
1228 # Keep only `sched_switch` events
1229 if event.name != 'sched_switch':
1232 # Keep only events which happened on CPU 0
1233 if event['cpu_id'] != 0:
1237 cur_ts = event.timestamp
1243 # Previous task command (short) name
1244 prev_comm = event['prev_comm']
1246 # Initialize entry in our dict if not yet done
1247 if prev_comm not in exec_times:
1248 exec_times[prev_comm] = 0
1250 # Compute previous command execution time
1251 diff = cur_ts - last_ts
1253 # Update execution time of this command
1254 exec_times[prev_comm] += diff
1256 # Update last timestamp
1260 for name, ns in exec_times.most_common(5):
1262 print('{:20}{} s'.format(name, s))
1267 if __name__ == '__main__':
1268 sys.exit(0 if top5proc() else 1)
1275 $ python3 top5proc.py /tmp/my-kernel-trace/kernel
1281 swapper/0 48.607245889 s
1282 chromium 7.192738188 s
1283 pavucontrol 0.709894415 s
1284 Compositor 0.660867933 s
1285 Xorg.bin 0.616753786 s
1288 Note that `swapper/0` is the "idle" process of CPU 0 on Linux; since we
1289 weren't using the CPU that much when tracing, its first position in the
1294 == [[understanding-lttng]]Core concepts
1296 From a user's perspective, the LTTng system is built on a few concepts,
1297 or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
1298 operates by sending commands to the <<lttng-sessiond,session daemon>>.
1299 Understanding how those objects relate to eachother is key in mastering
1302 The core concepts are:
1304 * <<tracing-session,Tracing session>>
1305 * <<domain,Tracing domain>>
1306 * <<channel,Channel and ring buffer>>
1307 * <<"event","Instrumentation point, event rule, event, and event record">>
1313 A _tracing session_ is a stateful dialogue between you and
1314 a <<lttng-sessiond,session daemon>>. You can
1315 <<creating-destroying-tracing-sessions,create a new tracing
1316 session>> with the `lttng create` command.
1318 Anything that you do when you control LTTng tracers happens within a
1319 tracing session. In particular, a tracing session:
1322 * Has its own set of trace files.
1323 * Has its own state of activity (started or stopped).
1324 * Has its own <<tracing-session-mode,mode>> (local, network streaming,
1326 * Has its own <<channel,channels>> which have their own
1327 <<event,event rules>>.
1330 .A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
1331 image::concepts.png[]
1333 Those attributes and objects are completely isolated between different
1336 A tracing session is analogous to a cash machine session:
1337 the operations you do on the banking system through the cash machine do
1338 not alter the data of other users of the same system. In the case of
1339 the cash machine, a session lasts as long as your bank card is inside.
1340 In the case of LTTng, a tracing session lasts from the `lttng create`
1341 command to the `lttng destroy` command.
1344 .Each Unix user has its own set of tracing sessions.
1345 image::many-sessions.png[]
1348 [[tracing-session-mode]]
1349 ==== Tracing session mode
1351 LTTng can send the generated trace data to different locations. The
1352 _tracing session mode_ dictates where to send it. The following modes
1353 are available in LTTng{nbsp}{revision}:
1356 LTTng writes the traces to the file system of the machine being traced
1359 Network streaming mode::
1360 LTTng sends the traces over the network to a
1361 <<lttng-relayd,relay daemon>> running on a remote system.
1364 LTTng does not write the traces by default. Instead, you can request
1365 LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
1366 current tracing buffers, and to write it to the target's file system
1367 or to send it over the network to a <<lttng-relayd,relay daemon>>
1368 running on a remote system.
1371 This mode is similar to the network streaming mode, but a live
1372 trace viewer can connect to the distant relay daemon to
1373 <<lttng-live,view event records as LTTng generates them>> by
1380 A _tracing domain_ is a namespace for event sources. A tracing domain
1381 has its own properties and features.
1383 There are currently five available tracing domains:
1387 * `java.util.logging` (JUL)
1391 You must specify a tracing domain when using some commands to avoid
1392 ambiguity. For example, since all the domains support named tracepoints
1393 as event sources (instrumentation points that you manually insert in the
1394 source code), you need to specify a tracing domain when
1395 <<enabling-disabling-events,creating an event rule>> because all the
1396 tracing domains could have tracepoints with the same names.
1398 Some features are reserved to specific tracing domains. Dynamic function
1399 entry and return instrumentation points, for example, are currently only
1400 supported in the Linux kernel tracing domain, but support for other
1401 tracing domains could be added in the future.
1403 You can create <<channel,channels>> in the Linux kernel and user space
1404 tracing domains. The other tracing domains have a single default
1409 === Channel and ring buffer
1411 A _channel_ is an object which is responsible for a set of ring buffers.
1412 Each ring buffer is divided into multiple sub-buffers. When an LTTng
1413 tracer emits an event, it can record it to one or more
1414 sub-buffers. The attributes of a channel determine what to do when
1415 there's no space left for a new event record because all sub-buffers
1416 are full, where to send a full sub-buffer, and other behaviours.
1418 A channel is always associated to a <<domain,tracing domain>>. The
1419 `java.util.logging` (JUL), log4j, and Python tracing domains each have
1420 a default channel which you cannot configure.
1422 A channel also owns <<event,event rules>>. When an LTTng tracer emits
1423 an event, it records it to the sub-buffers of all
1424 the enabled channels with a satisfied event rule, as long as those
1425 channels are part of active <<tracing-session,tracing sessions>>.
1428 [[channel-buffering-schemes]]
1429 ==== Per-user vs. per-process buffering schemes
1431 A channel has at least one ring buffer _per CPU_. LTTng always
1432 records an event to the ring buffer associated to the CPU on which it
1435 Two _buffering schemes_ are available when you
1436 <<enabling-disabling-channels,create a channel>> in the
1437 user space <<domain,tracing domain>>:
1439 Per-user buffering::
1440 Allocate one set of ring buffers--one per CPU--shared by all the
1441 instrumented processes of each Unix user.
1445 .Per-user buffering scheme.
1446 image::per-user-buffering.png[]
1449 Per-process buffering::
1450 Allocate one set of ring buffers--one per CPU--for each
1451 instrumented process.
1455 .Per-process buffering scheme.
1456 image::per-process-buffering.png[]
1459 The per-process buffering scheme tends to consume more memory than the
1460 per-user option because systems generally have more instrumented
1461 processes than Unix users running instrumented processes. However, the
1462 per-process buffering scheme ensures that one process having a high
1463 event throughput won't fill all the shared sub-buffers of the same
1466 The Linux kernel tracing domain has only one available buffering scheme
1467 which is to allocate a single set of ring buffers for the whole system.
1468 This scheme is similar to the per-user option, but with a single, global
1469 user "running" the kernel.
1472 [[channel-overwrite-mode-vs-discard-mode]]
1473 ==== Overwrite vs. discard event loss modes
1475 When an event occurs, LTTng records it to a specific sub-buffer (yellow
1476 arc in the following animation) of a specific channel's ring buffer.
1477 When there's no space left in a sub-buffer, the tracer marks it as
1478 consumable (red) and another, empty sub-buffer starts receiving the
1479 following event records. A <<lttng-consumerd,consumer daemon>>
1480 eventually consumes the marked sub-buffer (returns to white).
1483 [role="docsvg-channel-subbuf-anim"]
1488 In an ideal world, sub-buffers are consumed faster than they are filled,
1489 as is the case in the previous animation. In the real world,
1490 however, all sub-buffers can be full at some point, leaving no space to
1491 record the following events.
1493 By default, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when
1494 no empty sub-buffer is available, it is acceptable to lose event records
1495 when the alternative would be to cause substantial delays in the
1496 instrumented application's execution. LTTng privileges performance over
1497 integrity; it aims at perturbing the traced system as little as possible
1498 in order to make tracing of subtle race conditions and rare interrupt
1501 Starting from LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST,
1502 supports a _blocking mode_. See the <<blocking-timeout-example,blocking
1503 timeout example>> to learn how to use the blocking mode.
1505 When it comes to losing event records because no empty sub-buffer is
1506 available, or because the <<opt-blocking-timeout,blocking timeout>> is
1507 reached, the channel's _event loss mode_ determines what to do. The
1508 available event loss modes are:
1511 Drop the newest event records until a the tracer releases a
1514 This is the only available mode when you specify a
1515 <<opt-blocking-timeout,blocking timeout>>.
1518 Clear the sub-buffer containing the oldest event records and start
1519 writing the newest event records there.
1521 This mode is sometimes called _flight recorder mode_ because it's
1523 https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
1524 always keep a fixed amount of the latest data.
1526 Which mechanism you should choose depends on your context: prioritize
1527 the newest or the oldest event records in the ring buffer?
1529 Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
1530 as soon as a there's no space left for a new event record, whereas in
1531 discard mode, the tracer only discards the event record that doesn't
1534 In discard mode, LTTng increments a count of lost event records when an
1535 event record is lost and saves this count to the trace. In overwrite
1536 mode, since LTTng 2.8, LTTng increments a count of lost sub-buffers when
1537 a sub-buffer is lost and saves this count to the trace. In this mode,
1538 the exact number of lost event records in those lost sub-buffers is not
1539 saved to the trace. Trace analyses can use the trace's saved discarded
1540 event record and sub-buffer counts to decide whether or not to perform
1541 the analyses even if trace data is known to be missing.
1543 There are a few ways to decrease your probability of losing event
1545 <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
1546 how you can fine-une the sub-buffer count and size of a channel to
1547 virtually stop losing event records, though at the cost of greater
1551 [[channel-subbuf-size-vs-subbuf-count]]
1552 ==== Sub-buffer count and size
1554 When you <<enabling-disabling-channels,create a channel>>, you can
1555 set its number of sub-buffers and their size.
1557 Note that there is noticeable CPU overhead introduced when
1558 switching sub-buffers (marking a full one as consumable and switching
1559 to an empty one for the following events to be recorded). Knowing this,
1560 the following list presents a few practical situations along with how
1561 to configure the sub-buffer count and size for them:
1563 * **High event throughput**: In general, prefer bigger sub-buffers to
1564 lower the risk of losing event records.
1566 Having bigger sub-buffers also ensures a lower
1567 <<channel-switch-timer,sub-buffer switching frequency>>.
1569 The number of sub-buffers is only meaningful if you create the channel
1570 in overwrite mode: in this case, if a sub-buffer overwrite happens, the
1571 other sub-buffers are left unaltered.
1573 * **Low event throughput**: In general, prefer smaller sub-buffers
1574 since the risk of losing event records is low.
1576 Because events occur less frequently, the sub-buffer switching frequency
1577 should remain low and thus the tracer's overhead should not be a
1580 * **Low memory system**: If your target system has a low memory
1581 limit, prefer fewer first, then smaller sub-buffers.
1583 Even if the system is limited in memory, you want to keep the
1584 sub-buffers as big as possible to avoid a high sub-buffer switching
1587 Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
1588 which means event data is very compact. For example, the average
1589 LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
1590 sub-buffer size of 1{nbsp}MiB is considered big.
1592 The previous situations highlight the major trade-off between a few big
1593 sub-buffers and more, smaller sub-buffers: sub-buffer switching
1594 frequency vs. how much data is lost in overwrite mode. Assuming a
1595 constant event throughput and using the overwrite mode, the two
1596 following configurations have the same ring buffer total size:
1599 [role="docsvg-channel-subbuf-size-vs-count-anim"]
1604 * **2 sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
1605 switching frequency, but if a sub-buffer overwrite happens, half of
1606 the event records so far (4{nbsp}MiB) are definitely lost.
1607 * **8 sub-buffers of 1{nbsp}MiB each**: Expect 4{nbsp}times the tracer's
1608 overhead as the previous configuration, but if a sub-buffer
1609 overwrite happens, only the eighth of event records so far are
1612 In discard mode, the sub-buffers count parameter is pointless: use two
1613 sub-buffers and set their size according to the requirements of your
1617 [[channel-switch-timer]]
1618 ==== Switch timer period
1620 The _switch timer period_ is an important configurable attribute of
1621 a channel to ensure periodic sub-buffer flushing.
1623 When the _switch timer_ expires, a sub-buffer switch happens. You can
1624 set the switch timer period attribute when you
1625 <<enabling-disabling-channels,create a channel>> to ensure that event
1626 data is consumed and committed to trace files or to a distant relay
1627 daemon periodically in case of a low event throughput.
1630 [role="docsvg-channel-switch-timer"]
1635 This attribute is also convenient when you use big sub-buffers to cope
1636 with a sporadic high event throughput, even if the throughput is
1640 [[channel-read-timer]]
1641 ==== Read timer period
1643 By default, the LTTng tracers use a notification mechanism to signal a
1644 full sub-buffer so that a consumer daemon can consume it. When such
1645 notifications must be avoided, for example in real-time applications,
1646 you can use the channel's _read timer_ instead. When the read timer
1647 fires, the <<lttng-consumerd,consumer daemon>> checks for full,
1648 consumable sub-buffers.
1651 [[tracefile-rotation]]
1652 ==== Trace file count and size
1654 By default, trace files can grow as large as needed. You can set the
1655 maximum size of each trace file that a channel writes when you
1656 <<enabling-disabling-channels,create a channel>>. When the size of
1657 a trace file reaches the channel's fixed maximum size, LTTng creates
1658 another file to contain the next event records. LTTng appends a file
1659 count to each trace file name in this case.
1661 If you set the trace file size attribute when you create a channel, the
1662 maximum number of trace files that LTTng creates is _unlimited_ by
1663 default. To limit them, you can also set a maximum number of trace
1664 files. When the number of trace files reaches the channel's fixed
1665 maximum count, the oldest trace file is overwritten. This mechanism is
1666 called _trace file rotation_.
1670 === Instrumentation point, event rule, event, and event record
1672 An _event rule_ is a set of conditions which must be **all** satisfied
1673 for LTTng to record an occuring event.
1675 You set the conditions when you <<enabling-disabling-events,create
1678 You always attach an event rule to <<channel,channel>> when you create
1681 When an event passes the conditions of an event rule, LTTng records it
1682 in one of the attached channel's sub-buffers.
1684 The available conditions, as of LTTng{nbsp}{revision}, are:
1686 * The event rule _is enabled_.
1687 * The instrumentation point's type _is{nbsp}T_.
1688 * The instrumentation point's name (sometimes called _event name_)
1689 _matches{nbsp}N_, but _is not{nbsp}E_.
1690 * The instrumentation point's log level _is as severe as{nbsp}L_, or
1691 _is exactly{nbsp}L_.
1692 * The fields of the event's payload _satisfy_ a filter
1693 expression{nbsp}__F__.
1695 As you can see, all the conditions but the dynamic filter are related to
1696 the event rule's status or to the instrumentation point, not to the
1697 occurring events. This is why, without a filter, checking if an event
1698 passes an event rule is not a dynamic task: when you create or modify an
1699 event rule, all the tracers of its tracing domain enable or disable the
1700 instrumentation points themselves once. This is possible because the
1701 attributes of an instrumentation point (type, name, and log level) are
1702 defined statically. In other words, without a dynamic filter, the tracer
1703 _does not evaluate_ the arguments of an instrumentation point unless it
1704 matches an enabled event rule.
1706 Note that, for LTTng to record an event, the <<channel,channel>> to
1707 which a matching event rule is attached must also be enabled, and the
1708 tracing session owning this channel must be active.
1711 .Logical path from an instrumentation point to an event record.
1712 image::event-rule.png[]
1714 .Event, event record, or event rule?
1716 With so many similar terms, it's easy to get confused.
1718 An **event** is the consequence of the execution of an _instrumentation
1719 point_, like a tracepoint that you manually place in some source code,
1720 or a Linux kernel KProbe. An event is said to _occur_ at a specific
1721 time. Different actions can be taken upon the occurrence of an event,
1722 like record the event's payload to a buffer.
1724 An **event record** is the representation of an event in a sub-buffer. A
1725 tracer is responsible for capturing the payload of an event, current
1726 context variables, the event's ID, and the event's timestamp. LTTng
1727 can append this sub-buffer to a trace file.
1729 An **event rule** is a set of conditions which must all be satisfied for
1730 LTTng to record an occuring event. Events still occur without
1731 satisfying event rules, but LTTng does not record them.
1736 == Components of noch:{LTTng}
1738 The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
1739 to call LTTng a simple _tool_ since it is composed of multiple
1740 interacting components. This section describes those components,
1741 explains their respective roles, and shows how they connect together to
1742 form the LTTng ecosystem.
1744 The following diagram shows how the most important components of LTTng
1745 interact with user applications, the Linux kernel, and you:
1748 .Control and trace data paths between LTTng components.
1749 image::plumbing.png[]
1751 The LTTng project incorporates:
1753 * **LTTng-tools**: Libraries and command-line interface to
1754 control tracing sessions.
1755 ** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
1756 ** <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
1757 ** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
1758 ** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
1759 ** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
1760 * **LTTng-UST**: Libraries and Java/Python packages to trace user
1762 ** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
1763 headers to instrument and trace any native user application.
1764 ** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
1765 *** `liblttng-ust-libc-wrapper`
1766 *** `liblttng-ust-pthread-wrapper`
1767 *** `liblttng-ust-cyg-profile`
1768 *** `liblttng-ust-cyg-profile-fast`
1769 *** `liblttng-ust-dl`
1770 ** User space tracepoint provider source files generator command-line
1771 tool (man:lttng-gen-tp(1)).
1772 ** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
1773 Java applications using `java.util.logging` or
1774 Apache log4j 1.2 logging.
1775 ** <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
1776 Python applications using the standard `logging` package.
1777 * **LTTng-modules**: <<lttng-modules,Linux kernel modules>> to trace
1779 ** LTTng kernel tracer module.
1780 ** Tracing ring buffer kernel modules.
1781 ** Probe kernel modules.
1782 ** LTTng logger kernel module.
1786 === Tracing control command-line interface
1789 .The tracing control command-line interface.
1790 image::plumbing-lttng-cli.png[]
1792 The _man:lttng(1) command-line tool_ is the standard user interface to
1793 control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
1794 is part of LTTng-tools.
1796 The cmd:lttng tool is linked with
1797 <<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
1798 one or more <<lttng-sessiond,session daemons>> behind the scenes.
1800 The cmd:lttng tool has a Git-like interface:
1804 $ lttng <GENERAL OPTIONS> <COMMAND> <COMMAND OPTIONS>
1807 The <<controlling-tracing,Tracing control>> section explores the
1808 available features of LTTng using the cmd:lttng tool.
1811 [[liblttng-ctl-lttng]]
1812 === Tracing control library
1815 .The tracing control library.
1816 image::plumbing-liblttng-ctl.png[]
1818 The _LTTng control library_, `liblttng-ctl`, is used to communicate
1819 with a <<lttng-sessiond,session daemon>> using a C API that hides the
1820 underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.
1822 The <<lttng-cli,cmd:lttng command-line tool>>
1823 is linked with `liblttng-ctl`.
1825 You can use `liblttng-ctl` in C or $$C++$$ source code by including its
1830 #include <lttng/lttng.h>
1833 Some objects are referenced by name (C string), such as tracing
1834 sessions, but most of them require to create a handle first using
1835 `lttng_create_handle()`.
1837 The best available developer documentation for `liblttng-ctl` is, as of
1838 LTTng{nbsp}{revision}, its installed header files. Every function and
1839 structure is thoroughly documented.
1843 === User space tracing library
1846 .The user space tracing library.
1847 image::plumbing-liblttng-ust.png[]
1849 The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
1850 is the LTTng user space tracer. It receives commands from a
1851 <<lttng-sessiond,session daemon>>, for example to
1852 enable and disable specific instrumentation points, and writes event
1853 records to ring buffers shared with a
1854 <<lttng-consumerd,consumer daemon>>.
1855 `liblttng-ust` is part of LTTng-UST.
1857 Public C header files are installed beside `liblttng-ust` to
1858 instrument any <<c-application,C or $$C++$$ application>>.
1860 <<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
1861 packages, use their own library providing tracepoints which is
1862 linked with `liblttng-ust`.
1864 An application or library does not have to initialize `liblttng-ust`
1865 manually: its constructor does the necessary tasks to properly register
1866 to a session daemon. The initialization phase also enables the
1867 instrumentation points matching the <<event,event rules>> that you
1871 [[lttng-ust-agents]]
1872 === User space tracing agents
1875 .The user space tracing agents.
1876 image::plumbing-lttng-ust-agents.png[]
1878 The _LTTng-UST Java and Python agents_ are regular Java and Python
1879 packages which add LTTng tracing capabilities to the
1880 native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
1882 In the case of Java, the
1883 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
1884 core logging facilities] and
1885 https://logging.apache.org/log4j/1.2/[Apache log4j 1.2] are supported.
1886 Note that Apache Log4{nbsp}2 is not supported.
1888 In the case of Python, the standard
1889 https://docs.python.org/3/library/logging.html[`logging`] package
1890 is supported. Both Python 2 and Python 3 modules can import the
1891 LTTng-UST Python agent package.
1893 The applications using the LTTng-UST agents are in the
1894 `java.util.logging` (JUL),
1895 log4j, and Python <<domain,tracing domains>>.
1897 Both agents use the same mechanism to trace the log statements. When an
1898 agent is initialized, it creates a log handler that attaches to the root
1899 logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
1900 When the application executes a log statement, it is passed to the
1901 agent's log handler by the root logger. The agent's log handler calls a
1902 native function in a tracepoint provider package shared library linked
1903 with <<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
1904 other fields, like its logger name and its log level. This native
1905 function contains a user space instrumentation point, hence tracing the
1908 The log level condition of an
1909 <<event,event rule>> is considered when tracing
1910 a Java or a Python application, and it's compatible with the standard
1911 JUL, log4j, and Python log levels.
1915 === LTTng kernel modules
1918 .The LTTng kernel modules.
1919 image::plumbing-lttng-modules.png[]
1921 The _LTTng kernel modules_ are a set of Linux kernel modules
1922 which implement the kernel tracer of the LTTng project. The LTTng
1923 kernel modules are part of LTTng-modules.
1925 The LTTng kernel modules include:
1927 * A set of _probe_ modules.
1929 Each module attaches to a specific subsystem
1930 of the Linux kernel using its tracepoint instrument points. There are
1931 also modules to attach to the entry and return points of the Linux
1932 system call functions.
1934 * _Ring buffer_ modules.
1936 A ring buffer implementation is provided as kernel modules. The LTTng
1937 kernel tracer writes to the ring buffer; a
1938 <<lttng-consumerd,consumer daemon>> reads from the ring buffer.
1940 * The _LTTng kernel tracer_ module.
1941 * The _LTTng logger_ module.
1943 The LTTng logger module implements the special path:{/proc/lttng-logger}
1944 file so that any executable can generate LTTng events by opening and
1945 writing to this file.
1947 See <<proc-lttng-logger-abi,LTTng logger>>.
1949 Generally, you do not have to load the LTTng kernel modules manually
1950 (using man:modprobe(8), for example): a root <<lttng-sessiond,session
1951 daemon>> loads the necessary modules when starting. If you have extra
1952 probe modules, you can specify to load them to the session daemon on
1955 The LTTng kernel modules are installed in
1956 +/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
1957 the kernel release (see `uname --kernel-release`).
1964 .The session daemon.
1965 image::plumbing-sessiond.png[]
1967 The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
1968 managing tracing sessions and for controlling the various components of
1969 LTTng. The session daemon is part of LTTng-tools.
1971 The session daemon sends control requests to and receives control
1974 * The <<lttng-ust,user space tracing library>>.
1976 Any instance of the user space tracing library first registers to
1977 a session daemon. Then, the session daemon can send requests to
1978 this instance, such as:
1981 ** Get the list of tracepoints.
1982 ** Share an <<event,event rule>> so that the user space tracing library
1983 can enable or disable tracepoints. Amongst the possible conditions
1984 of an event rule is a filter expression which `liblttng-ust` evalutes
1985 when an event occurs.
1986 ** Share <<channel,channel>> attributes and ring buffer locations.
1989 The session daemon and the user space tracing library use a Unix
1990 domain socket for their communication.
1992 * The <<lttng-ust-agents,user space tracing agents>>.
1994 Any instance of a user space tracing agent first registers to
1995 a session daemon. Then, the session daemon can send requests to
1996 this instance, such as:
1999 ** Get the list of loggers.
2000 ** Enable or disable a specific logger.
2003 The session daemon and the user space tracing agent use a TCP connection
2004 for their communication.
2006 * The <<lttng-modules,LTTng kernel tracer>>.
2007 * The <<lttng-consumerd,consumer daemon>>.
2009 The session daemon sends requests to the consumer daemon to instruct
2010 it where to send the trace data streams, amongst other information.
2012 * The <<lttng-relayd,relay daemon>>.
2014 The session daemon receives commands from the
2015 <<liblttng-ctl-lttng,tracing control library>>.
2017 The root session daemon loads the appropriate
2018 <<lttng-modules,LTTng kernel modules>> on startup. It also spawns
2019 a <<lttng-consumerd,consumer daemon>> as soon as you create
2020 an <<event,event rule>>.
2022 The session daemon does not send and receive trace data: this is the
2023 role of the <<lttng-consumerd,consumer daemon>> and
2024 <<lttng-relayd,relay daemon>>. It does, however, generate the
2025 http://diamon.org/ctf/[CTF] metadata stream.
2027 Each Unix user can have its own session daemon instance. The
2028 tracing sessions managed by different session daemons are completely
2031 The root user's session daemon is the only one which is
2032 allowed to control the LTTng kernel tracer, and its spawned consumer
2033 daemon is the only one which is allowed to consume trace data from the
2034 LTTng kernel tracer. Note, however, that any Unix user which is a member
2035 of the <<tracing-group,tracing group>> is allowed
2036 to create <<channel,channels>> in the
2037 Linux kernel <<domain,tracing domain>>, and thus to trace the Linux
2040 The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
2041 session daemon when using its `create` command if none is currently
2042 running. You can also start the session daemon manually.
2049 .The consumer daemon.
2050 image::plumbing-consumerd.png[]
2052 The _consumer daemon_, cmd:lttng-consumerd, is a daemon which shares
2053 ring buffers with user applications or with the LTTng kernel modules to
2054 collect trace data and send it to some location (on disk or to a
2055 <<lttng-relayd,relay daemon>> over the network). The consumer daemon
2056 is part of LTTng-tools.
2058 You do not start a consumer daemon manually: a consumer daemon is always
2059 spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
2060 <<event,event rule>>, that is, before you start tracing. When you kill
2061 its owner session daemon, the consumer daemon also exits because it is
2062 the session daemon's child process. Command-line options of
2063 man:lttng-sessiond(8) target the consumer daemon process.
2065 There are up to two running consumer daemons per Unix user, whereas only
2066 one session daemon can run per user. This is because each process can be
2067 either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
2068 and 64-bit processes, it is more efficient to have separate
2069 corresponding 32-bit and 64-bit consumer daemons. The root user is an
2070 exception: it can have up to _three_ running consumer daemons: 32-bit
2071 and 64-bit instances for its user applications, and one more
2072 reserved for collecting kernel trace data.
2080 image::plumbing-relayd.png[]
2082 The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
2083 between remote session and consumer daemons, local trace files, and a
2084 remote live trace viewer. The relay daemon is part of LTTng-tools.
2086 The main purpose of the relay daemon is to implement a receiver of
2087 <<sending-trace-data-over-the-network,trace data over the network>>.
2088 This is useful when the target system does not have much file system
2089 space to record trace files locally.
2091 The relay daemon is also a server to which a
2092 <<lttng-live,live trace viewer>> can
2093 connect. The live trace viewer sends requests to the relay daemon to
2094 receive trace data as the target system emits events. The
2095 communication protocol is named _LTTng live_; it is used over TCP
2098 Note that you can start the relay daemon on the target system directly.
2099 This is the setup of choice when the use case is to view events as
2100 the target system emits them without the need of a remote system.
2104 == [[using-lttng]]Instrumentation
2106 There are many examples of tracing and monitoring in our everyday life:
2108 * You have access to real-time and historical weather reports and
2109 forecasts thanks to weather stations installed around the country.
2110 * You know your heart is safe thanks to an electrocardiogram.
2111 * You make sure not to drive your car too fast and to have enough fuel
2112 to reach your destination thanks to gauges visible on your dashboard.
2114 All the previous examples have something in common: they rely on
2115 **instruments**. Without the electrodes attached to the surface of your
2116 body's skin, cardiac monitoring is futile.
2118 LTTng, as a tracer, is no different from those real life examples. If
2119 you're about to trace a software system or, in other words, record its
2120 history of execution, you better have **instrumentation points** in the
2121 subject you're tracing, that is, the actual software.
2123 Various ways were developed to instrument a piece of software for LTTng
2124 tracing. The most straightforward one is to manually place
2125 instrumentation points, called _tracepoints_, in the software's source
2126 code. It is also possible to add instrumentation points dynamically in
2127 the Linux kernel <<domain,tracing domain>>.
2129 If you're only interested in tracing the Linux kernel, your
2130 instrumentation needs are probably already covered by LTTng's built-in
2131 <<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
2132 user application which is already instrumented for LTTng tracing.
2133 In such cases, you can skip this whole section and read the topics of
2134 the <<controlling-tracing,Tracing control>> section.
2136 Many methods are available to instrument a piece of software for LTTng
2139 * <<c-application,User space instrumentation for C and $$C++$$
2141 * <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
2142 * <<java-application,User space Java agent>>.
2143 * <<python-application,User space Python agent>>.
2144 * <<proc-lttng-logger-abi,LTTng logger>>.
2145 * <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
2149 === [[cxx-application]]User space instrumentation for C and $$C++$$ applications
2151 The procedure to instrument a C or $$C++$$ user application with
2152 the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
2154 . <<tracepoint-provider,Create the source files of a tracepoint provider
2156 . <<probing-the-application-source-code,Add tracepoints to
2157 the application's source code>>.
2158 . <<building-tracepoint-providers-and-user-application,Build and link
2159 a tracepoint provider package and the user application>>.
2161 If you need quick, man:printf(3)-like instrumentation, you can skip
2162 those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
2165 IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
2166 instrument a user application with `liblttng-ust`.
2169 [[tracepoint-provider]]
2170 ==== Create the source files of a tracepoint provider package
2172 A _tracepoint provider_ is a set of compiled functions which provide
2173 **tracepoints** to an application, the type of instrumentation point
2174 supported by LTTng-UST. Those functions can emit events with
2175 user-defined fields and serialize those events as event records to one
2176 or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
2177 macro, which you <<probing-the-application-source-code,insert in a user
2178 application's source code>>, calls those functions.
2180 A _tracepoint provider package_ is an object file (`.o`) or a shared
2181 library (`.so`) which contains one or more tracepoint providers.
2182 Its source files are:
2184 * One or more <<tpp-header,tracepoint provider header>> (`.h`).
2185 * A <<tpp-source,tracepoint provider package source>> (`.c`).
2187 A tracepoint provider package is dynamically linked with `liblttng-ust`,
2188 the LTTng user space tracer, at run time.
2191 .User application linked with `liblttng-ust` and containing a tracepoint provider.
2192 image::ust-app.png[]
2194 NOTE: If you need quick, man:printf(3)-like instrumentation, you can
2195 skip creating and using a tracepoint provider and use
2196 <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
2200 ===== Create a tracepoint provider header file template
2202 A _tracepoint provider header file_ contains the tracepoint
2203 definitions of a tracepoint provider.
2205 To create a tracepoint provider header file:
2207 . Start from this template:
2211 .Tracepoint provider header file template (`.h` file extension).
2213 #undef TRACEPOINT_PROVIDER
2214 #define TRACEPOINT_PROVIDER provider_name
2216 #undef TRACEPOINT_INCLUDE
2217 #define TRACEPOINT_INCLUDE "./tp.h"
2219 #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
2222 #include <lttng/tracepoint.h>
2225 * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
2226 * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
2231 #include <lttng/tracepoint-event.h>
2237 * `provider_name` with the name of your tracepoint provider.
2238 * `"tp.h"` with the name of your tracepoint provider header file.
2240 . Below the `#include <lttng/tracepoint.h>` line, put your
2241 <<defining-tracepoints,tracepoint definitions>>.
2243 Your tracepoint provider name must be unique amongst all the possible
2244 tracepoint provider names used on the same target system. We
2245 suggest to include the name of your project or company in the name,
2246 for example, `org_lttng_my_project_tpp`.
2248 TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
2249 this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
2250 write are the <<defining-tracepoints,tracepoint definitions>>.
2253 [[defining-tracepoints]]
2254 ===== Create a tracepoint definition
2256 A _tracepoint definition_ defines, for a given tracepoint:
2258 * Its **input arguments**. They are the macro parameters that the
2259 `tracepoint()` macro accepts for this particular tracepoint
2260 in the user application's source code.
2261 * Its **output event fields**. They are the sources of event fields
2262 that form the payload of any event that the execution of the
2263 `tracepoint()` macro emits for this particular tracepoint.
2265 You can create a tracepoint definition by using the
2266 `TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
2268 <<tpp-header,tracepoint provider header file template>>.
2270 The syntax of the `TRACEPOINT_EVENT()` macro is:
2273 .`TRACEPOINT_EVENT()` macro syntax.
2276 /* Tracepoint provider name */
2279 /* Tracepoint name */
2282 /* Input arguments */
2287 /* Output event fields */
2296 * `provider_name` with your tracepoint provider name.
2297 * `tracepoint_name` with your tracepoint name.
2298 * `arguments` with the <<tpp-def-input-args,input arguments>>.
2299 * `fields` with the <<tpp-def-output-fields,output event field>>
2302 This tracepoint emits events named `provider_name:tracepoint_name`.
2305 .Event name's length limitation
2307 The concatenation of the tracepoint provider name and the
2308 tracepoint name must not exceed **254 characters**. If it does, the
2309 instrumented application compiles and runs, but LTTng throws multiple
2310 warnings and you could experience serious issues.
2313 [[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is:
2316 .`TP_ARGS()` macro syntax.
2325 * `type` with the C type of the argument.
2326 * `arg_name` with the argument name.
2328 You can repeat `type` and `arg_name` up to 10 times to have
2329 more than one argument.
2331 .`TP_ARGS()` usage with three arguments.
2343 The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
2344 tracepoint definition with no input arguments.
2346 [[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
2347 `ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
2348 man:lttng-ust(3) for a complete description of the available `ctf_*()`
2349 macros. A `ctf_*()` macro specifies the type, size, and byte order of
2352 Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
2353 C expression that the tracer evalutes at the `tracepoint()` macro site
2354 in the application's source code. This expression provides a field's
2355 source of data. The argument expression can include input argument names
2356 listed in the `TP_ARGS()` macro.
2358 Each `ctf_*()` macro also takes a _field name_ parameter. Field names
2359 must be unique within a given tracepoint definition.
2361 Here's a complete tracepoint definition example:
2363 .Tracepoint definition.
2365 The following tracepoint definition defines a tracepoint which takes
2366 three input arguments and has four output event fields.
2370 #include "my-custom-structure.h"
2376 const struct my_custom_structure*, my_custom_structure,
2381 ctf_string(query_field, query)
2382 ctf_float(double, ratio_field, ratio)
2383 ctf_integer(int, recv_size, my_custom_structure->recv_size)
2384 ctf_integer(int, send_size, my_custom_structure->send_size)
2389 You can refer to this tracepoint definition with the `tracepoint()`
2390 macro in your application's source code like this:
2394 tracepoint(my_provider, my_tracepoint,
2395 my_structure, some_ratio, the_query);
2399 NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
2400 if they satisfy an enabled <<event,event rule>>.
2403 [[using-tracepoint-classes]]
2404 ===== Use a tracepoint class
2406 A _tracepoint class_ is a class of tracepoints which share the same
2407 output event field definitions. A _tracepoint instance_ is one
2408 instance of such a defined tracepoint class, with its own tracepoint
2411 The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
2412 shorthand which defines both a tracepoint class and a tracepoint
2413 instance at the same time.
2415 When you build a tracepoint provider package, the C or $$C++$$ compiler
2416 creates one serialization function for each **tracepoint class**. A
2417 serialization function is responsible for serializing the event fields
2418 of a tracepoint to a sub-buffer when tracing.
2420 For various performance reasons, when your situation requires multiple
2421 tracepoint definitions with different names, but with the same event
2422 fields, we recommend that you manually create a tracepoint class
2423 and instantiate as many tracepoint instances as needed. One positive
2424 effect of such a design, amongst other advantages, is that all
2425 tracepoint instances of the same tracepoint class reuse the same
2426 serialization function, thus reducing
2427 https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
2429 .Use a tracepoint class and tracepoint instances.
2431 Consider the following three tracepoint definitions:
2443 ctf_integer(int, userid, userid)
2444 ctf_integer(size_t, len, len)
2456 ctf_integer(int, userid, userid)
2457 ctf_integer(size_t, len, len)
2469 ctf_integer(int, userid, userid)
2470 ctf_integer(size_t, len, len)
2475 In this case, we create three tracepoint classes, with one implicit
2476 tracepoint instance for each of them: `get_account`, `get_settings`, and
2477 `get_transaction`. However, they all share the same event field names
2478 and types. Hence three identical, yet independent serialization
2479 functions are created when you build the tracepoint provider package.
2481 A better design choice is to define a single tracepoint class and three
2482 tracepoint instances:
2486 /* The tracepoint class */
2487 TRACEPOINT_EVENT_CLASS(
2488 /* Tracepoint provider name */
2491 /* Tracepoint class name */
2494 /* Input arguments */
2500 /* Output event fields */
2502 ctf_integer(int, userid, userid)
2503 ctf_integer(size_t, len, len)
2507 /* The tracepoint instances */
2508 TRACEPOINT_EVENT_INSTANCE(
2509 /* Tracepoint provider name */
2512 /* Tracepoint class name */
2515 /* Tracepoint name */
2518 /* Input arguments */
2524 TRACEPOINT_EVENT_INSTANCE(
2533 TRACEPOINT_EVENT_INSTANCE(
2546 [[assigning-log-levels]]
2547 ===== Assign a log level to a tracepoint definition
2549 You can assign an optional _log level_ to a
2550 <<defining-tracepoints,tracepoint definition>>.
2552 Assigning different levels of severity to tracepoint definitions can
2553 be useful: when you <<enabling-disabling-events,create an event rule>>,
2554 you can target tracepoints having a log level as severe as a specific
2557 The concept of LTTng-UST log levels is similar to the levels found
2558 in typical logging frameworks:
2560 * In a logging framework, the log level is given by the function
2561 or method name you use at the log statement site: `debug()`,
2562 `info()`, `warn()`, `error()`, and so on.
2563 * In LTTng-UST, you statically assign the log level to a tracepoint
2564 definition; any `tracepoint()` macro invocation which refers to
2565 this definition has this log level.
2567 You can assign a log level to a tracepoint definition with the
2568 `TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
2569 <<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
2570 <<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
2573 The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
2576 .`TRACEPOINT_LOGLEVEL()` macro syntax.
2578 TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
2583 * `provider_name` with the tracepoint provider name.
2584 * `tracepoint_name` with the tracepoint name.
2585 * `log_level` with the log level to assign to the tracepoint
2586 definition named `tracepoint_name` in the `provider_name`
2587 tracepoint provider.
2589 See man:lttng-ust(3) for a list of available log level names.
2591 .Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
2595 /* Tracepoint definition */
2604 ctf_integer(int, userid, userid)
2605 ctf_integer(size_t, len, len)
2609 /* Log level assignment */
2610 TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
2616 ===== Create a tracepoint provider package source file
2618 A _tracepoint provider package source file_ is a C source file which
2619 includes a <<tpp-header,tracepoint provider header file>> to expand its
2620 macros into event serialization and other functions.
2622 You can always use the following tracepoint provider package source
2626 .Tracepoint provider package source file template.
2628 #define TRACEPOINT_CREATE_PROBES
2633 Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
2634 header file>> name. You may also include more than one tracepoint
2635 provider header file here to create a tracepoint provider package
2636 holding more than one tracepoint providers.
2639 [[probing-the-application-source-code]]
2640 ==== Add tracepoints to an application's source code
2642 Once you <<tpp-header,create a tracepoint provider header file>>, you
2643 can use the `tracepoint()` macro in your application's
2644 source code to insert the tracepoints that this header
2645 <<defining-tracepoints,defines>>.
2647 The `tracepoint()` macro takes at least two parameters: the tracepoint
2648 provider name and the tracepoint name. The corresponding tracepoint
2649 definition defines the other parameters.
2651 .`tracepoint()` usage.
2653 The following <<defining-tracepoints,tracepoint definition>> defines a
2654 tracepoint which takes two input arguments and has two output event
2658 .Tracepoint provider header file.
2660 #include "my-custom-structure.h"
2667 const char*, cmd_name
2670 ctf_string(cmd_name, cmd_name)
2671 ctf_integer(int, number_of_args, argc)
2676 You can refer to this tracepoint definition with the `tracepoint()`
2677 macro in your application's source code like this:
2680 .Application's source file.
2684 int main(int argc, char* argv[])
2686 tracepoint(my_provider, my_tracepoint, argc, argv[0]);
2692 Note how the application's source code includes
2693 the tracepoint provider header file containing the tracepoint
2694 definitions to use, path:{tp.h}.
2697 .`tracepoint()` usage with a complex tracepoint definition.
2699 Consider this complex tracepoint definition, where multiple event
2700 fields refer to the same input arguments in their argument expression
2704 .Tracepoint provider header file.
2706 /* For `struct stat` */
2707 #include <sys/types.h>
2708 #include <sys/stat.h>
2720 ctf_integer(int, my_constant_field, 23 + 17)
2721 ctf_integer(int, my_int_arg_field, my_int_arg)
2722 ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
2723 ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
2724 my_str_arg[2] + my_str_arg[3])
2725 ctf_string(my_str_arg_field, my_str_arg)
2726 ctf_integer_hex(off_t, size_field, st->st_size)
2727 ctf_float(double, size_dbl_field, (double) st->st_size)
2728 ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
2729 size_t, strlen(my_str_arg) / 2)
2734 You can refer to this tracepoint definition with the `tracepoint()`
2735 macro in your application's source code like this:
2738 .Application's source file.
2740 #define TRACEPOINT_DEFINE
2747 stat("/etc/fstab", &s);
2748 tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
2754 If you look at the event record that LTTng writes when tracing this
2755 program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
2756 it should look like this:
2758 .Event record fields
2760 |Field's name |Field's value
2761 |`my_constant_field` |40
2762 |`my_int_arg_field` |23
2763 |`my_int_arg_field2` |529
2765 |`my_str_arg_field` |`Hello, World!`
2766 |`size_field` |0x12d
2767 |`size_dbl_field` |301.0
2768 |`half_my_str_arg_field` |`Hello,`
2772 Sometimes, the arguments you pass to `tracepoint()` are expensive to
2773 compute--they use the call stack, for example. To avoid this
2774 computation when the tracepoint is disabled, you can use the
2775 `tracepoint_enabled()` and `do_tracepoint()` macros.
2777 The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
2781 .`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
2783 tracepoint_enabled(provider_name, tracepoint_name)
2784 do_tracepoint(provider_name, tracepoint_name, ...)
2789 * `provider_name` with the tracepoint provider name.
2790 * `tracepoint_name` with the tracepoint name.
2792 `tracepoint_enabled()` returns a non-zero value if the tracepoint named
2793 `tracepoint_name` from the provider named `provider_name` is enabled
2796 `do_tracepoint()` is like `tracepoint()`, except that it doesn't check
2797 if the tracepoint is enabled. Using `tracepoint()` with
2798 `tracepoint_enabled()` is dangerous since `tracepoint()` also contains
2799 the `tracepoint_enabled()` check, thus a race condition is
2800 possible in this situation:
2803 .Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
2805 if (tracepoint_enabled(my_provider, my_tracepoint)) {
2806 stuff = prepare_stuff();
2809 tracepoint(my_provider, my_tracepoint, stuff);
2812 If the tracepoint is enabled after the condition, then `stuff` is not
2813 prepared: the emitted event will either contain wrong data, or the whole
2814 application could crash (segmentation fault, for example).
2816 NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
2817 `STAP_PROBEV()` call. If you need it, you must emit
2821 [[building-tracepoint-providers-and-user-application]]
2822 ==== Build and link a tracepoint provider package and an application
2824 Once you have one or more <<tpp-header,tracepoint provider header
2825 files>> and a <<tpp-source,tracepoint provider package source file>>,
2826 you can create the tracepoint provider package by compiling its source
2827 file. From here, multiple build and run scenarios are possible. The
2828 following table shows common application and library configurations
2829 along with the required command lines to achieve them.
2831 In the following diagrams, we use the following file names:
2834 Executable application.
2837 Application's object file.
2840 Tracepoint provider package object file.
2843 Tracepoint provider package archive file.
2846 Tracepoint provider package shared object file.
2849 User library object file.
2852 User library shared object file.
2854 We use the following symbols in the diagrams of table below:
2857 .Symbols used in the build scenario diagrams.
2858 image::ust-sit-symbols.png[]
2860 We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
2861 variable in the following instructions.
2863 [role="growable ust-scenarios",cols="asciidoc,asciidoc"]
2864 .Common tracepoint provider package scenarios.
2866 |Scenario |Instructions
2869 The instrumented application is statically linked with
2870 the tracepoint provider package object.
2872 image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
2875 include::../common/ust-sit-step-tp-o.txt[]
2877 To build the instrumented application:
2879 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2884 #define TRACEPOINT_DEFINE
2888 . Compile the application source file:
2897 . Build the application:
2902 $ gcc -o app app.o tpp.o -llttng-ust -ldl
2906 To run the instrumented application:
2908 * Start the application:
2918 The instrumented application is statically linked with the
2919 tracepoint provider package archive file.
2921 image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
2924 To create the tracepoint provider package archive file:
2926 . Compile the <<tpp-source,tracepoint provider package source file>>:
2935 . Create the tracepoint provider package archive file:
2940 $ ar rcs tpp.a tpp.o
2944 To build the instrumented application:
2946 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2951 #define TRACEPOINT_DEFINE
2955 . Compile the application source file:
2964 . Build the application:
2969 $ gcc -o app app.o tpp.a -llttng-ust -ldl
2973 To run the instrumented application:
2975 * Start the application:
2985 The instrumented application is linked with the tracepoint provider
2986 package shared object.
2988 image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
2991 include::../common/ust-sit-step-tp-so.txt[]
2993 To build the instrumented application:
2995 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3000 #define TRACEPOINT_DEFINE
3004 . Compile the application source file:
3013 . Build the application:
3018 $ gcc -o app app.o -ldl -L. -ltpp
3022 To run the instrumented application:
3024 * Start the application:
3034 The tracepoint provider package shared object is preloaded before the
3035 instrumented application starts.
3037 image::ust-sit+tp-so-preloaded+app-instrumented.png[]
3040 include::../common/ust-sit-step-tp-so.txt[]
3042 To build the instrumented application:
3044 . In path:{app.c}, before including path:{tpp.h}, add the
3050 #define TRACEPOINT_DEFINE
3051 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3055 . Compile the application source file:
3064 . Build the application:
3069 $ gcc -o app app.o -ldl
3073 To run the instrumented application with tracing support:
3075 * Preload the tracepoint provider package shared object and
3076 start the application:
3081 $ LD_PRELOAD=./libtpp.so ./app
3085 To run the instrumented application without tracing support:
3087 * Start the application:
3097 The instrumented application dynamically loads the tracepoint provider
3098 package shared object.
3100 See the <<dlclose-warning,warning about `dlclose()`>>.
3102 image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
3105 include::../common/ust-sit-step-tp-so.txt[]
3107 To build the instrumented application:
3109 . In path:{app.c}, before including path:{tpp.h}, add the
3115 #define TRACEPOINT_DEFINE
3116 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3120 . Compile the application source file:
3129 . Build the application:
3134 $ gcc -o app app.o -ldl
3138 To run the instrumented application:
3140 * Start the application:
3150 The application is linked with the instrumented user library.
3152 The instrumented user library is statically linked with the tracepoint
3153 provider package object file.
3155 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
3158 include::../common/ust-sit-step-tp-o-fpic.txt[]
3160 To build the instrumented user library:
3162 . In path:{emon.c}, before including path:{tpp.h}, add the
3168 #define TRACEPOINT_DEFINE
3172 . Compile the user library source file:
3177 $ gcc -I. -fpic -c emon.c
3181 . Build the user library shared object:
3186 $ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
3190 To build the application:
3192 . Compile the application source file:
3201 . Build the application:
3206 $ gcc -o app app.o -L. -lemon
3210 To run the application:
3212 * Start the application:
3222 The application is linked with the instrumented user library.
3224 The instrumented user library is linked with the tracepoint provider
3225 package shared object.
3227 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3230 include::../common/ust-sit-step-tp-so.txt[]
3232 To build the instrumented user library:
3234 . In path:{emon.c}, before including path:{tpp.h}, add the
3240 #define TRACEPOINT_DEFINE
3244 . Compile the user library source file:
3249 $ gcc -I. -fpic -c emon.c
3253 . Build the user library shared object:
3258 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3262 To build the application:
3264 . Compile the application source file:
3273 . Build the application:
3278 $ gcc -o app app.o -L. -lemon
3282 To run the application:
3284 * Start the application:
3294 The tracepoint provider package shared object is preloaded before the
3297 The application is linked with the instrumented user library.
3299 image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
3302 include::../common/ust-sit-step-tp-so.txt[]
3304 To build the instrumented user library:
3306 . In path:{emon.c}, before including path:{tpp.h}, add the
3312 #define TRACEPOINT_DEFINE
3313 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3317 . Compile the user library source file:
3322 $ gcc -I. -fpic -c emon.c
3326 . Build the user library shared object:
3331 $ gcc -shared -o libemon.so emon.o -ldl
3335 To build the application:
3337 . Compile the application source file:
3346 . Build the application:
3351 $ gcc -o app app.o -L. -lemon
3355 To run the application with tracing support:
3357 * Preload the tracepoint provider package shared object and
3358 start the application:
3363 $ LD_PRELOAD=./libtpp.so ./app
3367 To run the application without tracing support:
3369 * Start the application:
3379 The application is linked with the instrumented user library.
3381 The instrumented user library dynamically loads the tracepoint provider
3382 package shared object.
3384 See the <<dlclose-warning,warning about `dlclose()`>>.
3386 image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3389 include::../common/ust-sit-step-tp-so.txt[]
3391 To build the instrumented user library:
3393 . In path:{emon.c}, before including path:{tpp.h}, add the
3399 #define TRACEPOINT_DEFINE
3400 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3404 . Compile the user library source file:
3409 $ gcc -I. -fpic -c emon.c
3413 . Build the user library shared object:
3418 $ gcc -shared -o libemon.so emon.o -ldl
3422 To build the application:
3424 . Compile the application source file:
3433 . Build the application:
3438 $ gcc -o app app.o -L. -lemon
3442 To run the application:
3444 * Start the application:
3454 The application dynamically loads the instrumented user library.
3456 The instrumented user library is linked with the tracepoint provider
3457 package shared object.
3459 See the <<dlclose-warning,warning about `dlclose()`>>.
3461 image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3464 include::../common/ust-sit-step-tp-so.txt[]
3466 To build the instrumented user library:
3468 . In path:{emon.c}, before including path:{tpp.h}, add the
3474 #define TRACEPOINT_DEFINE
3478 . Compile the user library source file:
3483 $ gcc -I. -fpic -c emon.c
3487 . Build the user library shared object:
3492 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3496 To build the application:
3498 . Compile the application source file:
3507 . Build the application:
3512 $ gcc -o app app.o -ldl -L. -lemon
3516 To run the application:
3518 * Start the application:
3528 The application dynamically loads the instrumented user library.
3530 The instrumented user library dynamically loads the tracepoint provider
3531 package shared object.
3533 See the <<dlclose-warning,warning about `dlclose()`>>.
3535 image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3538 include::../common/ust-sit-step-tp-so.txt[]
3540 To build the instrumented user library:
3542 . In path:{emon.c}, before including path:{tpp.h}, add the
3548 #define TRACEPOINT_DEFINE
3549 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3553 . Compile the user library source file:
3558 $ gcc -I. -fpic -c emon.c
3562 . Build the user library shared object:
3567 $ gcc -shared -o libemon.so emon.o -ldl
3571 To build the application:
3573 . Compile the application source file:
3582 . Build the application:
3587 $ gcc -o app app.o -ldl -L. -lemon
3591 To run the application:
3593 * Start the application:
3603 The tracepoint provider package shared object is preloaded before the
3606 The application dynamically loads the instrumented user library.
3608 image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
3611 include::../common/ust-sit-step-tp-so.txt[]
3613 To build the instrumented user library:
3615 . In path:{emon.c}, before including path:{tpp.h}, add the
3621 #define TRACEPOINT_DEFINE
3622 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3626 . Compile the user library source file:
3631 $ gcc -I. -fpic -c emon.c
3635 . Build the user library shared object:
3640 $ gcc -shared -o libemon.so emon.o -ldl
3644 To build the application:
3646 . Compile the application source file:
3655 . Build the application:
3660 $ gcc -o app app.o -L. -lemon
3664 To run the application with tracing support:
3666 * Preload the tracepoint provider package shared object and
3667 start the application:
3672 $ LD_PRELOAD=./libtpp.so ./app
3676 To run the application without tracing support:
3678 * Start the application:
3688 The application is statically linked with the tracepoint provider
3689 package object file.
3691 The application is linked with the instrumented user library.
3693 image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
3696 include::../common/ust-sit-step-tp-o.txt[]
3698 To build the instrumented user library:
3700 . In path:{emon.c}, before including path:{tpp.h}, add the
3706 #define TRACEPOINT_DEFINE
3710 . Compile the user library source file:
3715 $ gcc -I. -fpic -c emon.c
3719 . Build the user library shared object:
3724 $ gcc -shared -o libemon.so emon.o
3728 To build the application:
3730 . Compile the application source file:
3739 . Build the application:
3744 $ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
3748 To run the instrumented application:
3750 * Start the application:
3760 The application is statically linked with the tracepoint provider
3761 package object file.
3763 The application dynamically loads the instrumented user library.
3765 image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
3768 include::../common/ust-sit-step-tp-o.txt[]
3770 To build the application:
3772 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3777 #define TRACEPOINT_DEFINE
3781 . Compile the application source file:
3790 . Build the application:
3795 $ gcc -Wl,--export-dynamic -o app app.o tpp.o \
3800 The `--export-dynamic` option passed to the linker is necessary for the
3801 dynamically loaded library to ``see'' the tracepoint symbols defined in
3804 To build the instrumented user library:
3806 . Compile the user library source file:
3811 $ gcc -I. -fpic -c emon.c
3815 . Build the user library shared object:
3820 $ gcc -shared -o libemon.so emon.o
3824 To run the application:
3826 * Start the application:
3838 .Do not use man:dlclose(3) on a tracepoint provider package
3840 Never use man:dlclose(3) on any shared object which:
3842 * Is linked with, statically or dynamically, a tracepoint provider
3844 * Calls man:dlopen(3) itself to dynamically open a tracepoint provider
3845 package shared object.
3847 This is currently considered **unsafe** due to a lack of reference
3848 counting from LTTng-UST to the shared object.
3850 A known workaround (available since glibc 2.2) is to use the
3851 `RTLD_NODELETE` flag when calling man:dlopen(3) initially. This has the
3852 effect of not unloading the loaded shared object, even if man:dlclose(3)
3855 You can also preload the tracepoint provider package shared object with
3856 the env:LD_PRELOAD environment variable to overcome this limitation.
3860 [[using-lttng-ust-with-daemons]]
3861 ===== Use noch:{LTTng-UST} with daemons
3863 If your instrumented application calls man:fork(2), man:clone(2),
3864 or BSD's man:rfork(2), without a following man:exec(3)-family
3865 system call, you must preload the path:{liblttng-ust-fork.so} shared
3866 object when you start the application.
3870 $ LD_PRELOAD=liblttng-ust-fork.so ./my-app
3873 If your tracepoint provider package is
3874 a shared library which you also preload, you must put both
3875 shared objects in env:LD_PRELOAD:
3879 $ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
3885 ===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them
3887 If your instrumented application closes one or more file descriptors
3888 which it did not open itself, you must preload the
3889 path:{liblttng-ust-fd.so} shared object when you start the application:
3893 $ LD_PRELOAD=liblttng-ust-fd.so ./my-app
3896 Typical use cases include closing all the file descriptors after
3897 man:fork(2) or man:rfork(2) and buggy applications doing
3901 [[lttng-ust-pkg-config]]
3902 ===== Use noch:{pkg-config}
3904 On some distributions, LTTng-UST ships with a
3905 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
3906 metadata file. If this is your case, then you can use cmd:pkg-config to
3907 build an application on the command line:
3911 $ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
3915 [[instrumenting-32-bit-app-on-64-bit-system]]
3916 ===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
3918 In order to trace a 32-bit application running on a 64-bit system,
3919 LTTng must use a dedicated 32-bit
3920 <<lttng-consumerd,consumer daemon>>.
3922 The following steps show how to build and install a 32-bit consumer
3923 daemon, which is _not_ part of the default 64-bit LTTng build, how to
3924 build and install the 32-bit LTTng-UST libraries, and how to build and
3925 link an instrumented 32-bit application in that context.
3927 To build a 32-bit instrumented application for a 64-bit target system,
3928 assuming you have a fresh target system with no installed Userspace RCU
3931 . Download, build, and install a 32-bit version of Userspace RCU:
3936 $ cd $(mktemp -d) &&
3937 wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
3938 tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
3939 cd userspace-rcu-0.9.* &&
3940 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
3942 sudo make install &&
3947 . Using your distribution's package manager, or from source, install
3948 the following 32-bit versions of the following dependencies of
3949 LTTng-tools and LTTng-UST:
3952 * https://sourceforge.net/projects/libuuid/[libuuid]
3953 * http://directory.fsf.org/wiki/Popt[popt]
3954 * http://www.xmlsoft.org/[libxml2]
3957 . Download, build, and install a 32-bit version of the latest
3958 LTTng-UST{nbsp}{revision}:
3963 $ cd $(mktemp -d) &&
3964 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.10.tar.bz2 &&
3965 tar -xf lttng-ust-latest-2.10.tar.bz2 &&
3966 cd lttng-ust-2.10.* &&
3967 ./configure --libdir=/usr/local/lib32 \
3968 CFLAGS=-m32 CXXFLAGS=-m32 \
3969 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
3971 sudo make install &&
3978 Depending on your distribution,
3979 32-bit libraries could be installed at a different location than
3980 `/usr/lib32`. For example, Debian is known to install
3981 some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
3983 In this case, make sure to set `LDFLAGS` to all the
3984 relevant 32-bit library paths, for example:
3988 $ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
3992 . Download the latest LTTng-tools{nbsp}{revision}, build, and install
3993 the 32-bit consumer daemon:
3998 $ cd $(mktemp -d) &&
3999 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.10.tar.bz2 &&
4000 tar -xf lttng-tools-latest-2.10.tar.bz2 &&
4001 cd lttng-tools-2.10.* &&
4002 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
4003 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
4004 --disable-bin-lttng --disable-bin-lttng-crash \
4005 --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
4007 cd src/bin/lttng-consumerd &&
4008 sudo make install &&
4013 . From your distribution or from source,
4014 <<installing-lttng,install>> the 64-bit versions of
4015 LTTng-UST and Userspace RCU.
4016 . Download, build, and install the 64-bit version of the
4017 latest LTTng-tools{nbsp}{revision}:
4022 $ cd $(mktemp -d) &&
4023 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.10.tar.bz2 &&
4024 tar -xf lttng-tools-latest-2.10.tar.bz2 &&
4025 cd lttng-tools-2.10.* &&
4026 ./configure --with-consumerd32-libdir=/usr/local/lib32 \
4027 --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
4029 sudo make install &&
4034 . Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
4035 when linking your 32-bit application:
4038 -m32 -L/usr/lib32 -L/usr/local/lib32 \
4039 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
4042 For example, let's rebuild the quick start example in
4043 <<tracing-your-own-user-application,Trace a user application>> as an
4044 instrumented 32-bit application:
4049 $ gcc -m32 -c -I. hello-tp.c
4050 $ gcc -m32 -c hello.c
4051 $ gcc -m32 -o hello hello.o hello-tp.o \
4052 -L/usr/lib32 -L/usr/local/lib32 \
4053 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
4058 No special action is required to execute the 32-bit application and
4059 to trace it: use the command-line man:lttng(1) tool as usual.
4066 man:tracef(3) is a small LTTng-UST API designed for quick,
4067 man:printf(3)-like instrumentation without the burden of
4068 <<tracepoint-provider,creating>> and
4069 <<building-tracepoint-providers-and-user-application,building>>
4070 a tracepoint provider package.
4072 To use `tracef()` in your application:
4074 . In the C or C++ source files where you need to use `tracef()`,
4075 include `<lttng/tracef.h>`:
4080 #include <lttng/tracef.h>
4084 . In the application's source code, use `tracef()` like you would use
4092 tracef("my message: %d (%s)", my_integer, my_string);
4098 . Link your application with `liblttng-ust`:
4103 $ gcc -o app app.c -llttng-ust
4107 To trace the events that `tracef()` calls emit:
4109 * <<enabling-disabling-events,Create an event rule>> which matches the
4110 `lttng_ust_tracef:*` event name:
4115 $ lttng enable-event --userspace 'lttng_ust_tracef:*'
4120 .Limitations of `tracef()`
4122 The `tracef()` utility function was developed to make user space tracing
4123 super simple, albeit with notable disadvantages compared to
4124 <<defining-tracepoints,user-defined tracepoints>>:
4126 * All the emitted events have the same tracepoint provider and
4127 tracepoint names, respectively `lttng_ust_tracef` and `event`.
4128 * There is no static type checking.
4129 * The only event record field you actually get, named `msg`, is a string
4130 potentially containing the values you passed to `tracef()`
4131 using your own format string. This also means that you cannot filter
4132 events with a custom expression at run time because there are no
4134 * Since `tracef()` uses the C standard library's man:vasprintf(3)
4135 function behind the scenes to format the strings at run time, its
4136 expected performance is lower than with user-defined tracepoints,
4137 which do not require a conversion to a string.
4139 Taking this into consideration, `tracef()` is useful for some quick
4140 prototyping and debugging, but you should not consider it for any
4141 permanent and serious applicative instrumentation.
4147 ==== Use `tracelog()`
4149 The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
4150 the difference that it accepts an additional log level parameter.
4152 The goal of `tracelog()` is to ease the migration from logging to
4155 To use `tracelog()` in your application:
4157 . In the C or C++ source files where you need to use `tracelog()`,
4158 include `<lttng/tracelog.h>`:
4163 #include <lttng/tracelog.h>
4167 . In the application's source code, use `tracelog()` like you would use
4168 man:printf(3), except for the first parameter which is the log
4176 tracelog(TRACE_WARNING, "my message: %d (%s)",
4177 my_integer, my_string);
4183 See man:lttng-ust(3) for a list of available log level names.
4185 . Link your application with `liblttng-ust`:
4190 $ gcc -o app app.c -llttng-ust
4194 To trace the events that `tracelog()` calls emit with a log level
4195 _as severe as_ a specific log level:
4197 * <<enabling-disabling-events,Create an event rule>> which matches the
4198 `lttng_ust_tracelog:*` event name and a minimum level
4204 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
4205 --loglevel=TRACE_WARNING
4209 To trace the events that `tracelog()` calls emit with a
4210 _specific log level_:
4212 * Create an event rule which matches the `lttng_ust_tracelog:*`
4213 event name and a specific log level:
4218 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
4219 --loglevel-only=TRACE_INFO
4224 [[prebuilt-ust-helpers]]
4225 === Prebuilt user space tracing helpers
4227 The LTTng-UST package provides a few helpers in the form or preloadable
4228 shared objects which automatically instrument system functions and
4231 The helper shared objects are normally found in dir:{/usr/lib}. If you
4232 built LTTng-UST <<building-from-source,from source>>, they are probably
4233 located in dir:{/usr/local/lib}.
4235 The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
4238 path:{liblttng-ust-libc-wrapper.so}::
4239 path:{liblttng-ust-pthread-wrapper.so}::
4240 <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
4241 memory and POSIX threads function tracing>>.
4243 path:{liblttng-ust-cyg-profile.so}::
4244 path:{liblttng-ust-cyg-profile-fast.so}::
4245 <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
4247 path:{liblttng-ust-dl.so}::
4248 <<liblttng-ust-dl,Dynamic linker tracing>>.
4250 To use a user space tracing helper with any user application:
4252 * Preload the helper shared object when you start the application:
4257 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
4261 You can preload more than one helper:
4266 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
4272 [[liblttng-ust-libc-pthread-wrapper]]
4273 ==== Instrument C standard library memory and POSIX threads functions
4275 The path:{liblttng-ust-libc-wrapper.so} and
4276 path:{liblttng-ust-pthread-wrapper.so} helpers
4277 add instrumentation to some C standard library and POSIX
4281 .Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
4283 |TP provider name |TP name |Instrumented function
4285 .6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
4286 |`calloc` |man:calloc(3)
4287 |`realloc` |man:realloc(3)
4288 |`free` |man:free(3)
4289 |`memalign` |man:memalign(3)
4290 |`posix_memalign` |man:posix_memalign(3)
4294 .Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
4296 |TP provider name |TP name |Instrumented function
4298 .4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
4299 |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
4300 |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
4301 |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
4304 When you preload the shared object, it replaces the functions listed
4305 in the previous tables by wrappers which contain tracepoints and call
4306 the replaced functions.
4309 [[liblttng-ust-cyg-profile]]
4310 ==== Instrument function entry and exit
4312 The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
4313 to the entry and exit points of functions.
4315 man:gcc(1) and man:clang(1) have an option named
4316 https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
4317 which generates instrumentation calls for entry and exit to functions.
4318 The LTTng-UST function tracing helpers,
4319 path:{liblttng-ust-cyg-profile.so} and
4320 path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
4321 to add tracepoints to the two generated functions (which contain
4322 `cyg_profile` in their names, hence the helper's name).
4324 To use the LTTng-UST function tracing helper, the source files to
4325 instrument must be built using the `-finstrument-functions` compiler
4328 There are two versions of the LTTng-UST function tracing helper:
4330 * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
4331 that you should only use when it can be _guaranteed_ that the
4332 complete event stream is recorded without any lost event record.
4333 Any kind of duplicate information is left out.
4335 Assuming no event record is lost, having only the function addresses on
4336 entry is enough to create a call graph, since an event record always
4337 contains the ID of the CPU that generated it.
4339 You can use a tool like man:addr2line(1) to convert function addresses
4340 back to source file names and line numbers.
4342 * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
4343 which also works in use cases where event records might get discarded or
4344 not recorded from application startup.
4345 In these cases, the trace analyzer needs more information to be
4346 able to reconstruct the program flow.
4348 See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
4349 points of this helper.
4351 All the tracepoints that this helper provides have the
4352 log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
4354 TIP: It's sometimes a good idea to limit the number of source files that
4355 you compile with the `-finstrument-functions` option to prevent LTTng
4356 from writing an excessive amount of trace data at run time. When using
4357 man:gcc(1), you can use the
4358 `-finstrument-functions-exclude-function-list` option to avoid
4359 instrument entries and exits of specific function names.
4364 ==== Instrument the dynamic linker
4366 The path:{liblttng-ust-dl.so} helper adds instrumentation to the
4367 man:dlopen(3) and man:dlclose(3) function calls.
4369 See man:lttng-ust-dl(3) to learn more about the instrumentation points
4374 [[java-application]]
4375 === User space Java agent
4377 You can instrument any Java application which uses one of the following
4380 * The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
4381 (JUL) core logging facilities.
4382 * http://logging.apache.org/log4j/1.2/[**Apache log4j 1.2**], since
4383 LTTng 2.6. Note that Apache Log4j{nbsp}2 is not supported.
4386 .LTTng-UST Java agent imported by a Java application.
4387 image::java-app.png[]
4389 Note that the methods described below are new in LTTng{nbsp}{revision}.
4390 Previous LTTng versions use another technique.
4392 NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
4393 and https://ci.lttng.org/[continuous integration], thus this version is
4394 directly supported. However, the LTTng-UST Java agent is also tested
4395 with OpenJDK{nbsp}7.
4400 ==== Use the LTTng-UST Java agent for `java.util.logging`
4402 To use the LTTng-UST Java agent in a Java application which uses
4403 `java.util.logging` (JUL):
4405 . In the Java application's source code, import the LTTng-UST
4406 log handler package for `java.util.logging`:
4411 import org.lttng.ust.agent.jul.LttngLogHandler;
4415 . Create an LTTng-UST JUL log handler:
4420 Handler lttngUstLogHandler = new LttngLogHandler();
4424 . Add this handler to the JUL loggers which should emit LTTng events:
4429 Logger myLogger = Logger.getLogger("some-logger");
4431 myLogger.addHandler(lttngUstLogHandler);
4435 . Use `java.util.logging` log statements and configuration as usual.
4436 The loggers with an attached LTTng-UST log handler can emit
4439 . Before exiting the application, remove the LTTng-UST log handler from
4440 the loggers attached to it and call its `close()` method:
4445 myLogger.removeHandler(lttngUstLogHandler);
4446 lttngUstLogHandler.close();
4450 This is not strictly necessary, but it is recommended for a clean
4451 disposal of the handler's resources.
4453 . Include the LTTng-UST Java agent's common and JUL-specific JAR files,
4454 path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
4456 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4457 path] when you build the Java application.
4459 The JAR files are typically located in dir:{/usr/share/java}.
4461 IMPORTANT: The LTTng-UST Java agent must be
4462 <<installing-lttng,installed>> for the logging framework your
4465 .Use the LTTng-UST Java agent for `java.util.logging`.
4470 import java.io.IOException;
4471 import java.util.logging.Handler;
4472 import java.util.logging.Logger;
4473 import org.lttng.ust.agent.jul.LttngLogHandler;
4477 private static final int answer = 42;
4479 public static void main(String[] argv) throws Exception
4482 Logger logger = Logger.getLogger("jello");
4484 // Create an LTTng-UST log handler
4485 Handler lttngUstLogHandler = new LttngLogHandler();
4487 // Add the LTTng-UST log handler to our logger
4488 logger.addHandler(lttngUstLogHandler);
4491 logger.info("some info");
4492 logger.warning("some warning");
4494 logger.finer("finer information; the answer is " + answer);
4496 logger.severe("error!");
4498 // Not mandatory, but cleaner
4499 logger.removeHandler(lttngUstLogHandler);
4500 lttngUstLogHandler.close();
4509 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4512 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4513 <<enabling-disabling-events,create an event rule>> matching the
4514 `jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
4519 $ lttng enable-event --jul jello
4523 Run the compiled class:
4527 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4530 <<basic-tracing-session-control,Stop tracing>> and inspect the
4540 In the resulting trace, an <<event,event record>> generated by a Java
4541 application using `java.util.logging` is named `lttng_jul:event` and
4542 has the following fields:
4545 Log record's message.
4551 Name of the class in which the log statement was executed.
4554 Name of the method in which the log statement was executed.
4557 Logging time (timestamp in milliseconds).
4560 Log level integer value.
4563 ID of the thread in which the log statement was executed.
4565 You can use the opt:lttng-enable-event(1):--loglevel or
4566 opt:lttng-enable-event(1):--loglevel-only option of the
4567 man:lttng-enable-event(1) command to target a range of JUL log levels
4568 or a specific JUL log level.
4573 ==== Use the LTTng-UST Java agent for Apache log4j
4575 To use the LTTng-UST Java agent in a Java application which uses
4578 . In the Java application's source code, import the LTTng-UST
4579 log appender package for Apache log4j:
4584 import org.lttng.ust.agent.log4j.LttngLogAppender;
4588 . Create an LTTng-UST log4j log appender:
4593 Appender lttngUstLogAppender = new LttngLogAppender();
4597 . Add this appender to the log4j loggers which should emit LTTng events:
4602 Logger myLogger = Logger.getLogger("some-logger");
4604 myLogger.addAppender(lttngUstLogAppender);
4608 . Use Apache log4j log statements and configuration as usual. The
4609 loggers with an attached LTTng-UST log appender can emit LTTng events.
4611 . Before exiting the application, remove the LTTng-UST log appender from
4612 the loggers attached to it and call its `close()` method:
4617 myLogger.removeAppender(lttngUstLogAppender);
4618 lttngUstLogAppender.close();
4622 This is not strictly necessary, but it is recommended for a clean
4623 disposal of the appender's resources.
4625 . Include the LTTng-UST Java agent's common and log4j-specific JAR
4626 files, path:{lttng-ust-agent-common.jar} and
4627 path:{lttng-ust-agent-log4j.jar}, in the
4628 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4629 path] when you build the Java application.
4631 The JAR files are typically located in dir:{/usr/share/java}.
4633 IMPORTANT: The LTTng-UST Java agent must be
4634 <<installing-lttng,installed>> for the logging framework your
4637 .Use the LTTng-UST Java agent for Apache log4j.
4642 import org.apache.log4j.Appender;
4643 import org.apache.log4j.Logger;
4644 import org.lttng.ust.agent.log4j.LttngLogAppender;
4648 private static final int answer = 42;
4650 public static void main(String[] argv) throws Exception
4653 Logger logger = Logger.getLogger("jello");
4655 // Create an LTTng-UST log appender
4656 Appender lttngUstLogAppender = new LttngLogAppender();
4658 // Add the LTTng-UST log appender to our logger
4659 logger.addAppender(lttngUstLogAppender);
4662 logger.info("some info");
4663 logger.warn("some warning");
4665 logger.debug("debug information; the answer is " + answer);
4667 logger.fatal("error!");
4669 // Not mandatory, but cleaner
4670 logger.removeAppender(lttngUstLogAppender);
4671 lttngUstLogAppender.close();
4677 Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
4682 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
4685 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4686 <<enabling-disabling-events,create an event rule>> matching the
4687 `jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
4692 $ lttng enable-event --log4j jello
4696 Run the compiled class:
4700 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
4703 <<basic-tracing-session-control,Stop tracing>> and inspect the
4713 In the resulting trace, an <<event,event record>> generated by a Java
4714 application using log4j is named `lttng_log4j:event` and
4715 has the following fields:
4718 Log record's message.
4724 Name of the class in which the log statement was executed.
4727 Name of the method in which the log statement was executed.
4730 Name of the file in which the executed log statement is located.
4733 Line number at which the log statement was executed.
4739 Log level integer value.
4742 Name of the Java thread in which the log statement was executed.
4744 You can use the opt:lttng-enable-event(1):--loglevel or
4745 opt:lttng-enable-event(1):--loglevel-only option of the
4746 man:lttng-enable-event(1) command to target a range of Apache log4j log levels
4747 or a specific log4j log level.
4751 [[java-application-context]]
4752 ==== Provide application-specific context fields in a Java application
4754 A Java application-specific context field is a piece of state provided
4755 by the application which <<adding-context,you can add>>, using the
4756 man:lttng-add-context(1) command, to each <<event,event record>>
4757 produced by the log statements of this application.
4759 For example, a given object might have a current request ID variable.
4760 You can create a context information retriever for this object and
4761 assign a name to this current request ID. You can then, using the
4762 man:lttng-add-context(1) command, add this context field by name to
4763 the JUL or log4j <<channel,channel>>.
4765 To provide application-specific context fields in a Java application:
4767 . In the Java application's source code, import the LTTng-UST
4768 Java agent context classes and interfaces:
4773 import org.lttng.ust.agent.context.ContextInfoManager;
4774 import org.lttng.ust.agent.context.IContextInfoRetriever;
4778 . Create a context information retriever class, that is, a class which
4779 implements the `IContextInfoRetriever` interface:
4784 class MyContextInfoRetriever implements IContextInfoRetriever
4787 public Object retrieveContextInfo(String key)
4789 if (key.equals("intCtx")) {
4791 } else if (key.equals("strContext")) {
4792 return "context value!";
4801 This `retrieveContextInfo()` method is the only member of the
4802 `IContextInfoRetriever` interface. Its role is to return the current
4803 value of a state by name to create a context field. The names of the
4804 context fields and which state variables they return depends on your
4807 All primitive types and objects are supported as context fields.
4808 When `retrieveContextInfo()` returns an object, the context field
4809 serializer calls its `toString()` method to add a string field to
4810 event records. The method can also return `null`, which means that
4811 no context field is available for the required name.
4813 . Register an instance of your context information retriever class to
4814 the context information manager singleton:
4819 IContextInfoRetriever cir = new MyContextInfoRetriever();
4820 ContextInfoManager cim = ContextInfoManager.getInstance();
4821 cim.registerContextInfoRetriever("retrieverName", cir);
4825 . Before exiting the application, remove your context information
4826 retriever from the context information manager singleton:
4831 ContextInfoManager cim = ContextInfoManager.getInstance();
4832 cim.unregisterContextInfoRetriever("retrieverName");
4836 This is not strictly necessary, but it is recommended for a clean
4837 disposal of some manager's resources.
4839 . Build your Java application with LTTng-UST Java agent support as
4840 usual, following the procedure for either the <<jul,JUL>> or
4841 <<log4j,Apache log4j>> framework.
4844 .Provide application-specific context fields in a Java application.
4849 import java.util.logging.Handler;
4850 import java.util.logging.Logger;
4851 import org.lttng.ust.agent.jul.LttngLogHandler;
4852 import org.lttng.ust.agent.context.ContextInfoManager;
4853 import org.lttng.ust.agent.context.IContextInfoRetriever;
4857 // Our context information retriever class
4858 private static class MyContextInfoRetriever
4859 implements IContextInfoRetriever
4862 public Object retrieveContextInfo(String key) {
4863 if (key.equals("intCtx")) {
4865 } else if (key.equals("strContext")) {
4866 return "context value!";
4873 private static final int answer = 42;
4875 public static void main(String args[]) throws Exception
4877 // Get the context information manager instance
4878 ContextInfoManager cim = ContextInfoManager.getInstance();
4880 // Create and register our context information retriever
4881 IContextInfoRetriever cir = new MyContextInfoRetriever();
4882 cim.registerContextInfoRetriever("myRetriever", cir);
4885 Logger logger = Logger.getLogger("jello");
4887 // Create an LTTng-UST log handler
4888 Handler lttngUstLogHandler = new LttngLogHandler();
4890 // Add the LTTng-UST log handler to our logger
4891 logger.addHandler(lttngUstLogHandler);
4894 logger.info("some info");
4895 logger.warning("some warning");
4897 logger.finer("finer information; the answer is " + answer);
4899 logger.severe("error!");
4901 // Not mandatory, but cleaner
4902 logger.removeHandler(lttngUstLogHandler);
4903 lttngUstLogHandler.close();
4904 cim.unregisterContextInfoRetriever("myRetriever");
4913 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4916 <<creating-destroying-tracing-sessions,Create a tracing session>>
4917 and <<enabling-disabling-events,create an event rule>> matching the
4923 $ lttng enable-event --jul jello
4926 <<adding-context,Add the application-specific context fields>> to the
4931 $ lttng add-context --jul --type='$app.myRetriever:intCtx'
4932 $ lttng add-context --jul --type='$app.myRetriever:strContext'
4935 <<basic-tracing-session-control,Start tracing>>:
4942 Run the compiled class:
4946 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4949 <<basic-tracing-session-control,Stop tracing>> and inspect the
4961 [[python-application]]
4962 === User space Python agent
4964 You can instrument a Python 2 or Python 3 application which uses the
4965 standard https://docs.python.org/3/library/logging.html[`logging`]
4968 Each log statement emits an LTTng event once the
4969 application module imports the
4970 <<lttng-ust-agents,LTTng-UST Python agent>> package.
4973 .A Python application importing the LTTng-UST Python agent.
4974 image::python-app.png[]
4976 To use the LTTng-UST Python agent:
4978 . In the Python application's source code, import the LTTng-UST Python
4988 The LTTng-UST Python agent automatically adds its logging handler to the
4989 root logger at import time.
4991 Any log statement that the application executes before this import does
4992 not emit an LTTng event.
4994 IMPORTANT: The LTTng-UST Python agent must be
4995 <<installing-lttng,installed>>.
4997 . Use log statements and logging configuration as usual.
4998 Since the LTTng-UST Python agent adds a handler to the _root_
4999 logger, you can trace any log statement from any logger.
5001 .Use the LTTng-UST Python agent.
5012 logging.basicConfig()
5013 logger = logging.getLogger('my-logger')
5016 logger.debug('debug message')
5017 logger.info('info message')
5018 logger.warn('warn message')
5019 logger.error('error message')
5020 logger.critical('critical message')
5024 if __name__ == '__main__':
5028 NOTE: `logging.basicConfig()`, which adds to the root logger a basic
5029 logging handler which prints to the standard error stream, is not
5030 strictly required for LTTng-UST tracing to work, but in versions of
5031 Python preceding 3.2, you could see a warning message which indicates
5032 that no handler exists for the logger `my-logger`.
5034 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5035 <<enabling-disabling-events,create an event rule>> matching the
5036 `my-logger` Python logger, and <<basic-tracing-session-control,start
5042 $ lttng enable-event --python my-logger
5046 Run the Python script:
5053 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5063 In the resulting trace, an <<event,event record>> generated by a Python
5064 application is named `lttng_python:event` and has the following fields:
5067 Logging time (string).
5070 Log record's message.
5076 Name of the function in which the log statement was executed.
5079 Line number at which the log statement was executed.
5082 Log level integer value.
5085 ID of the Python thread in which the log statement was executed.
5088 Name of the Python thread in which the log statement was executed.
5090 You can use the opt:lttng-enable-event(1):--loglevel or
5091 opt:lttng-enable-event(1):--loglevel-only option of the
5092 man:lttng-enable-event(1) command to target a range of Python log levels
5093 or a specific Python log level.
5095 When an application imports the LTTng-UST Python agent, the agent tries
5096 to register to a <<lttng-sessiond,session daemon>>. Note that you must
5097 <<start-sessiond,start the session daemon>> _before_ you run the Python
5098 application. If a session daemon is found, the agent tries to register
5099 to it during 5{nbsp}seconds, after which the application continues
5100 without LTTng tracing support. You can override this timeout value with
5101 the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
5104 If the session daemon stops while a Python application with an imported
5105 LTTng-UST Python agent runs, the agent retries to connect and to
5106 register to a session daemon every 3{nbsp}seconds. You can override this
5107 delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
5112 [[proc-lttng-logger-abi]]
5115 The `lttng-tracer` Linux kernel module, part of
5116 <<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
5117 path:{/proc/lttng-logger} when it's loaded. Any application can write
5118 text data to this file to emit an LTTng event.
5121 .An application writes to the LTTng logger file to emit an LTTng event.
5122 image::lttng-logger.png[]
5124 The LTTng logger is the quickest method--not the most efficient,
5125 however--to add instrumentation to an application. It is designed
5126 mostly to instrument shell scripts:
5130 $ echo "Some message, some $variable" > /proc/lttng-logger
5133 Any event that the LTTng logger emits is named `lttng_logger` and
5134 belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
5135 other instrumentation points in the kernel tracing domain, **any Unix
5136 user** can <<enabling-disabling-events,create an event rule>> which
5137 matches its event name, not only the root user or users in the
5138 <<tracing-group,tracing group>>.
5140 To use the LTTng logger:
5142 * From any application, write text data to the path:{/proc/lttng-logger}
5145 The `msg` field of `lttng_logger` event records contains the
5148 NOTE: The maximum message length of an LTTng logger event is
5149 1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
5150 than one event to contain the remaining data.
5152 You should not use the LTTng logger to trace a user application which
5153 can be instrumented in a more efficient way, namely:
5155 * <<c-application,C and $$C++$$ applications>>.
5156 * <<java-application,Java applications>>.
5157 * <<python-application,Python applications>>.
5159 .Use the LTTng logger.
5164 echo 'Hello, World!' > /proc/lttng-logger
5166 df --human-readable --print-type / > /proc/lttng-logger
5169 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5170 <<enabling-disabling-events,create an event rule>> matching the
5171 `lttng_logger` Linux kernel tracepoint, and
5172 <<basic-tracing-session-control,start tracing>>:
5177 $ lttng enable-event --kernel lttng_logger
5181 Run the Bash script:
5188 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5199 [[instrumenting-linux-kernel]]
5200 === LTTng kernel tracepoints
5202 NOTE: This section shows how to _add_ instrumentation points to the
5203 Linux kernel. The kernel's subsystems are already thoroughly
5204 instrumented at strategic places for LTTng when you
5205 <<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
5209 There are two methods to instrument the Linux kernel:
5211 . <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
5212 tracepoint which uses the `TRACE_EVENT()` API.
5214 Choose this if you want to instrumentation a Linux kernel tree with an
5215 instrumentation point compatible with ftrace, perf, and SystemTap.
5217 . Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
5218 instrument an out-of-tree kernel module.
5220 Choose this if you don't need ftrace, perf, or SystemTap support.
5224 [[linux-add-lttng-layer]]
5225 ==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
5227 This section shows how to add an LTTng layer to existing ftrace
5228 instrumentation using the `TRACE_EVENT()` API.
5230 This section does not document the `TRACE_EVENT()` macro. You can
5231 read the following articles to learn more about this API:
5233 * http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part 1)]
5234 * http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part 2)]
5235 * http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part 3)]
5237 The following procedure assumes that your ftrace tracepoints are
5238 correctly defined in their own header and that they are created in
5239 one source file using the `CREATE_TRACE_POINTS` definition.
5241 To add an LTTng layer over an existing ftrace tracepoint:
5243 . Make sure the following kernel configuration options are
5249 * `CONFIG_HIGH_RES_TIMERS`
5250 * `CONFIG_TRACEPOINTS`
5253 . Build the Linux source tree with your custom ftrace tracepoints.
5254 . Boot the resulting Linux image on your target system.
5256 Confirm that the tracepoints exist by looking for their names in the
5257 dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
5258 is your subsystem's name.
5260 . Get a copy of the latest LTTng-modules{nbsp}{revision}:
5265 $ cd $(mktemp -d) &&
5266 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.10.tar.bz2 &&
5267 tar -xf lttng-modules-latest-2.10.tar.bz2 &&
5268 cd lttng-modules-2.10.*
5272 . In dir:{instrumentation/events/lttng-module}, relative to the root
5273 of the LTTng-modules source tree, create a header file named
5274 +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
5275 LTTng-modules tracepoint definitions using the LTTng-modules
5278 Start with this template:
5282 .path:{instrumentation/events/lttng-module/my_subsys.h}
5285 #define TRACE_SYSTEM my_subsys
5287 #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
5288 #define _LTTNG_MY_SUBSYS_H
5290 #include "../../../probes/lttng-tracepoint-event.h"
5291 #include <linux/tracepoint.h>
5293 LTTNG_TRACEPOINT_EVENT(
5295 * Format is identical to TRACE_EVENT()'s version for the three
5296 * following macro parameters:
5299 TP_PROTO(int my_int, const char *my_string),
5300 TP_ARGS(my_int, my_string),
5302 /* LTTng-modules specific macros */
5304 ctf_integer(int, my_int_field, my_int)
5305 ctf_string(my_bar_field, my_bar)
5309 #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
5311 #include "../../../probes/define_trace.h"
5315 The entries in the `TP_FIELDS()` section are the list of fields for the
5316 LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
5317 ftrace's `TRACE_EVENT()` macro.
5319 See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
5320 complete description of the available `ctf_*()` macros.
5322 . Create the LTTng-modules probe's kernel module C source file,
5323 +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
5328 .path:{probes/lttng-probe-my-subsys.c}
5330 #include <linux/module.h>
5331 #include "../lttng-tracer.h"
5334 * Build-time verification of mismatch between mainline
5335 * TRACE_EVENT() arguments and the LTTng-modules adaptation
5336 * layer LTTNG_TRACEPOINT_EVENT() arguments.
5338 #include <trace/events/my_subsys.h>
5340 /* Create LTTng tracepoint probes */
5341 #define LTTNG_PACKAGE_BUILD
5342 #define CREATE_TRACE_POINTS
5343 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
5345 #include "../instrumentation/events/lttng-module/my_subsys.h"
5347 MODULE_LICENSE("GPL and additional rights");
5348 MODULE_AUTHOR("Your name <your-email>");
5349 MODULE_DESCRIPTION("LTTng my_subsys probes");
5350 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
5351 __stringify(LTTNG_MODULES_MINOR_VERSION) "."
5352 __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
5353 LTTNG_MODULES_EXTRAVERSION);
5357 . Edit path:{probes/KBuild} and add your new kernel module object
5358 next to the existing ones:
5362 .path:{probes/KBuild}
5366 obj-m += lttng-probe-module.o
5367 obj-m += lttng-probe-power.o
5369 obj-m += lttng-probe-my-subsys.o
5375 . Build and install the LTTng kernel modules:
5380 $ make KERNELDIR=/path/to/linux
5381 # make modules_install && depmod -a
5385 Replace `/path/to/linux` with the path to the Linux source tree where
5386 you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro.
5388 Note that you can also use the
5389 <<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
5390 instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
5391 C code that need to be executed before the event fields are recorded.
5393 The best way to learn how to use the previous LTTng-modules macros is to
5394 inspect the existing LTTng-modules tracepoint definitions in the
5395 dir:{instrumentation/events/lttng-module} header files. Compare them
5396 with the Linux kernel mainline versions in the
5397 dir:{include/trace/events} directory of the Linux source tree.
5401 [[lttng-tracepoint-event-code]]
5402 ===== Use custom C code to access the data for tracepoint fields
5404 Although we recommended to always use the
5405 <<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
5406 the arguments and fields of an LTTng-modules tracepoint when possible,
5407 sometimes you need a more complex process to access the data that the
5408 tracer records as event record fields. In other words, you need local
5409 variables and multiple C{nbsp}statements instead of simple
5410 argument-based expressions that you pass to the
5411 <<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
5413 You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
5414 `LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
5415 a block of C{nbsp}code to be executed before LTTng records the fields.
5416 The structure of this macro is:
5419 .`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
5421 LTTNG_TRACEPOINT_EVENT_CODE(
5423 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5424 * version for the following three macro parameters:
5427 TP_PROTO(int my_int, const char *my_string),
5428 TP_ARGS(my_int, my_string),
5430 /* Declarations of custom local variables */
5433 unsigned long b = 0;
5434 const char *name = "(undefined)";
5435 struct my_struct *my_struct;
5439 * Custom code which uses both tracepoint arguments
5440 * (in TP_ARGS()) and local variables (in TP_locvar()).
5442 * Local variables are actually members of a structure pointed
5443 * to by the special variable tp_locvar.
5447 tp_locvar->a = my_int + 17;
5448 tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
5449 tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
5450 tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
5451 put_my_struct(tp_locvar->my_struct);
5460 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5461 * version for this, except that tp_locvar members can be
5462 * used in the argument expression parameters of
5463 * the ctf_*() macros.
5466 ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
5467 ctf_integer(int, my_struct_a, tp_locvar->a)
5468 ctf_string(my_string_field, my_string)
5469 ctf_string(my_struct_name, tp_locvar->name)
5474 IMPORTANT: The C code defined in `TP_code()` must not have any side
5475 effects when executed. In particular, the code must not allocate
5476 memory or get resources without deallocating this memory or putting
5477 those resources afterwards.
5480 [[instrumenting-linux-kernel-tracing]]
5481 ==== Load and unload a custom probe kernel module
5483 You must load a <<lttng-adaptation-layer,created LTTng-modules probe
5484 kernel module>> in the kernel before it can emit LTTng events.
5486 To load the default probe kernel modules and a custom probe kernel
5489 * Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
5490 probe modules to load when starting a root <<lttng-sessiond,session
5494 .Load the `my_subsys`, `usb`, and the default probe modules.
5498 # lttng-sessiond --extra-kmod-probes=my_subsys,usb
5503 You only need to pass the subsystem name, not the whole kernel module
5506 To load _only_ a given custom probe kernel module:
5508 * Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
5509 modules to load when starting a root session daemon:
5512 .Load only the `my_subsys` and `usb` probe modules.
5516 # lttng-sessiond --kmod-probes=my_subsys,usb
5521 To confirm that a probe module is loaded:
5528 $ lsmod | grep lttng_probe_usb
5532 To unload the loaded probe modules:
5534 * Kill the session daemon with `SIGTERM`:
5539 # pkill lttng-sessiond
5543 You can also use man:modprobe(8)'s `--remove` option if the session
5544 daemon terminates abnormally.
5547 [[controlling-tracing]]
5550 Once an application or a Linux kernel is
5551 <<instrumenting,instrumented>> for LTTng tracing,
5554 This section is divided in topics on how to use the various
5555 <<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
5556 command-line tool>>, to _control_ the LTTng daemons and tracers.
5558 NOTE: In the following subsections, we refer to an man:lttng(1) command
5559 using its man page name. For example, instead of _Run the `create`
5560 command to..._, we use _Run the man:lttng-create(1) command to..._.
5564 === Start a session daemon
5566 In some situations, you need to run a <<lttng-sessiond,session daemon>>
5567 (man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
5570 You will see the following error when you run a command while no session
5574 Error: No session daemon is available
5577 The only command that automatically runs a session daemon is
5578 man:lttng-create(1), which you use to
5579 <<creating-destroying-tracing-sessions,create a tracing session>>. While
5580 this is most of the time the first operation that you do, sometimes it's
5581 not. Some examples are:
5583 * <<list-instrumentation-points,List the available instrumentation points>>.
5584 * <<saving-loading-tracing-session,Load a tracing session configuration>>.
5586 [[tracing-group]] Each Unix user must have its own running session
5587 daemon to trace user applications. The session daemon that the root user
5588 starts is the only one allowed to control the LTTng kernel tracer. Users
5589 that are part of the _tracing group_ can control the root session
5590 daemon. The default tracing group name is `tracing`; you can set it to
5591 something else with the opt:lttng-sessiond(8):--group option when you
5592 start the root session daemon.
5594 To start a user session daemon:
5596 * Run man:lttng-sessiond(8):
5601 $ lttng-sessiond --daemonize
5605 To start the root session daemon:
5607 * Run man:lttng-sessiond(8) as the root user:
5612 # lttng-sessiond --daemonize
5616 In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
5617 start the session daemon in foreground.
5619 To stop a session daemon, use man:kill(1) on its process ID (standard
5622 Note that some Linux distributions could manage the LTTng session daemon
5623 as a service. In this case, you should use the service manager to
5624 start, restart, and stop session daemons.
5627 [[creating-destroying-tracing-sessions]]
5628 === Create and destroy a tracing session
5630 Almost all the LTTng control operations happen in the scope of
5631 a <<tracing-session,tracing session>>, which is the dialogue between the
5632 <<lttng-sessiond,session daemon>> and you.
5634 To create a tracing session with a generated name:
5636 * Use the man:lttng-create(1) command:
5645 The created tracing session's name is `auto` followed by the
5648 To create a tracing session with a specific name:
5650 * Use the optional argument of the man:lttng-create(1) command:
5655 $ lttng create my-session
5659 Replace `my-session` with the specific tracing session name.
5661 LTTng appends the creation date to the created tracing session's name.
5663 LTTng writes the traces of a tracing session in
5664 +$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
5665 name of the tracing session. Note that the env:LTTNG_HOME environment
5666 variable defaults to `$HOME` if not set.
5668 To output LTTng traces to a non-default location:
5670 * Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
5675 $ lttng create my-session --output=/tmp/some-directory
5679 You may create as many tracing sessions as you wish.
5681 To list all the existing tracing sessions for your Unix user:
5683 * Use the man:lttng-list(1) command:
5692 When you create a tracing session, it is set as the _current tracing
5693 session_. The following man:lttng(1) commands operate on the current
5694 tracing session when you don't specify one:
5696 [role="list-3-cols"]
5713 To change the current tracing session:
5715 * Use the man:lttng-set-session(1) command:
5720 $ lttng set-session new-session
5724 Replace `new-session` by the name of the new current tracing session.
5726 When you are done tracing in a given tracing session, you can destroy
5727 it. This operation frees the resources taken by the tracing session
5728 to destroy; it does not destroy the trace data that LTTng wrote for
5729 this tracing session.
5731 To destroy the current tracing session:
5733 * Use the man:lttng-destroy(1) command:
5743 [[list-instrumentation-points]]
5744 === List the available instrumentation points
5746 The <<lttng-sessiond,session daemon>> can query the running instrumented
5747 user applications and the Linux kernel to get a list of available
5748 instrumentation points. For the Linux kernel <<domain,tracing domain>>,
5749 they are tracepoints and system calls. For the user space tracing
5750 domain, they are tracepoints. For the other tracing domains, they are
5753 To list the available instrumentation points:
5755 * Use the man:lttng-list(1) command with the requested tracing domain's
5759 * opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
5760 must be a root user, or it must be a member of the
5761 <<tracing-group,tracing group>>).
5762 * opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
5763 kernel system calls (your Unix user must be a root user, or it must be
5764 a member of the tracing group).
5765 * opt:lttng-list(1):--userspace: user space tracepoints.
5766 * opt:lttng-list(1):--jul: `java.util.logging` loggers.
5767 * opt:lttng-list(1):--log4j: Apache log4j loggers.
5768 * opt:lttng-list(1):--python: Python loggers.
5771 .List the available user space tracepoints.
5775 $ lttng list --userspace
5779 .List the available Linux kernel system call tracepoints.
5783 $ lttng list --kernel --syscall
5788 [[enabling-disabling-events]]
5789 === Create and enable an event rule
5791 Once you <<creating-destroying-tracing-sessions,create a tracing
5792 session>>, you can create <<event,event rules>> with the
5793 man:lttng-enable-event(1) command.
5795 You specify each condition with a command-line option. The available
5796 condition options are shown in the following table.
5798 [role="growable",cols="asciidoc,asciidoc,default"]
5799 .Condition command-line options for the man:lttng-enable-event(1) command.
5801 |Option |Description |Applicable tracing domains
5807 . +--probe=__ADDR__+
5808 . +--function=__ADDR__+
5811 Instead of using the default _tracepoint_ instrumentation type, use:
5813 . A Linux system call.
5814 . A Linux https://lwn.net/Articles/132196/[KProbe] (symbol or address).
5815 . The entry and return points of a Linux function (symbol or address).
5819 |First positional argument.
5822 Tracepoint or system call name. In the case of a Linux KProbe or
5823 function, this is a custom name given to the event rule. With the
5824 JUL, log4j, and Python domains, this is a logger name.
5826 With a tracepoint, logger, or system call name, you can use the special
5827 `*` globbing character to match anything (for example, `sched_*`,
5835 . +--loglevel=__LEVEL__+
5836 . +--loglevel-only=__LEVEL__+
5839 . Match only tracepoints or log statements with a logging level at
5840 least as severe as +__LEVEL__+.
5841 . Match only tracepoints or log statements with a logging level
5842 equal to +__LEVEL__+.
5844 See man:lttng-enable-event(1) for the list of available logging level
5847 |User space, JUL, log4j, and Python.
5849 |+--exclude=__EXCLUSIONS__+
5852 When you use a `*` character at the end of the tracepoint or logger
5853 name (first positional argument), exclude the specific names in the
5854 comma-delimited list +__EXCLUSIONS__+.
5857 User space, JUL, log4j, and Python.
5859 |+--filter=__EXPR__+
5862 Match only events which satisfy the expression +__EXPR__+.
5864 See man:lttng-enable-event(1) to learn more about the syntax of a
5871 You attach an event rule to a <<channel,channel>> on creation. If you do
5872 not specify the channel with the opt:lttng-enable-event(1):--channel
5873 option, and if the event rule to create is the first in its
5874 <<domain,tracing domain>> for a given tracing session, then LTTng
5875 creates a _default channel_ for you. This default channel is reused in
5876 subsequent invocations of the man:lttng-enable-event(1) command for the
5877 same tracing domain.
5879 An event rule is always enabled at creation time.
5881 The following examples show how you can combine the previous
5882 command-line options to create simple to more complex event rules.
5884 .Create an event rule targetting a Linux kernel tracepoint (default channel).
5888 $ lttng enable-event --kernel sched_switch
5892 .Create an event rule matching four Linux kernel system calls (default channel).
5896 $ lttng enable-event --kernel --syscall open,write,read,close
5900 .Create event rules matching tracepoints with filter expressions (default channel).
5904 $ lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
5909 $ lttng enable-event --kernel --all \
5910 --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
5915 $ lttng enable-event --jul my_logger \
5916 --filter='$app.retriever:cur_msg_id > 3'
5919 IMPORTANT: Make sure to always quote the filter string when you
5920 use man:lttng(1) from a shell.
5923 .Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
5927 $ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
5930 IMPORTANT: Make sure to always quote the wildcard character when you
5931 use man:lttng(1) from a shell.
5934 .Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
5938 $ lttng enable-event --python my-app.'*' \
5939 --exclude='my-app.module,my-app.hello'
5943 .Create an event rule matching any Apache log4j logger with a specific log level (default channel).
5947 $ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
5951 .Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
5955 $ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
5959 The event rules of a given channel form a whitelist: as soon as an
5960 emitted event passes one of them, LTTng can record the event. For
5961 example, an event named `my_app:my_tracepoint` emitted from a user space
5962 tracepoint with a `TRACE_ERROR` log level passes both of the following
5967 $ lttng enable-event --userspace my_app:my_tracepoint
5968 $ lttng enable-event --userspace my_app:my_tracepoint \
5969 --loglevel=TRACE_INFO
5972 The second event rule is redundant: the first one includes
5976 [[disable-event-rule]]
5977 === Disable an event rule
5979 To disable an event rule that you <<enabling-disabling-events,created>>
5980 previously, use the man:lttng-disable-event(1) command. This command
5981 disables _all_ the event rules (of a given tracing domain and channel)
5982 which match an instrumentation point. The other conditions are not
5983 supported as of LTTng{nbsp}{revision}.
5985 The LTTng tracer does not record an emitted event which passes
5986 a _disabled_ event rule.
5988 .Disable an event rule matching a Python logger (default channel).
5992 $ lttng disable-event --python my-logger
5996 .Disable an event rule matching all `java.util.logging` loggers (default channel).
6000 $ lttng disable-event --jul '*'
6004 .Disable _all_ the event rules of the default channel.
6006 The opt:lttng-disable-event(1):--all-events option is not, like the
6007 opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
6008 equivalent of the event name `*` (wildcard): it disables _all_ the event
6009 rules of a given channel.
6013 $ lttng disable-event --jul --all-events
6017 NOTE: You cannot delete an event rule once you create it.
6021 === Get the status of a tracing session
6023 To get the status of the current tracing session, that is, its
6024 parameters, its channels, event rules, and their attributes:
6026 * Use the man:lttng-status(1) command:
6036 To get the status of any tracing session:
6038 * Use the man:lttng-list(1) command with the tracing session's name:
6043 $ lttng list my-session
6047 Replace `my-session` with the desired tracing session's name.
6050 [[basic-tracing-session-control]]
6051 === Start and stop a tracing session
6053 Once you <<creating-destroying-tracing-sessions,create a tracing
6055 <<enabling-disabling-events,create one or more event rules>>,
6056 you can start and stop the tracers for this tracing session.
6058 To start tracing in the current tracing session:
6060 * Use the man:lttng-start(1) command:
6069 LTTng is very flexible: you can launch user applications before
6070 or after the you start the tracers. The tracers only record the events
6071 if they pass enabled event rules and if they occur while the tracers are
6074 To stop tracing in the current tracing session:
6076 * Use the man:lttng-stop(1) command:
6085 If there were <<channel-overwrite-mode-vs-discard-mode,lost event
6086 records>> or lost sub-buffers since the last time you ran
6087 man:lttng-start(1), warnings are printed when you run the
6088 man:lttng-stop(1) command.
6091 [[enabling-disabling-channels]]
6092 === Create a channel
6094 Once you create a tracing session, you can create a <<channel,channel>>
6095 with the man:lttng-enable-channel(1) command.
6097 Note that LTTng automatically creates a default channel when, for a
6098 given <<domain,tracing domain>>, no channels exist and you
6099 <<enabling-disabling-events,create>> the first event rule. This default
6100 channel is named `channel0` and its attributes are set to reasonable
6101 values. Therefore, you only need to create a channel when you need
6102 non-default attributes.
6104 You specify each non-default channel attribute with a command-line
6105 option when you use the man:lttng-enable-channel(1) command. The
6106 available command-line options are:
6108 [role="growable",cols="asciidoc,asciidoc"]
6109 .Command-line options for the man:lttng-enable-channel(1) command.
6111 |Option |Description
6117 <<channel-overwrite-mode-vs-discard-mode,event loss mode>> instead of
6118 the default _discard_ mode.
6120 |`--buffers-pid` (user space tracing domain only)
6123 Use the per-process <<channel-buffering-schemes,buffering scheme>>
6124 instead of the default per-user buffering scheme.
6126 |+--subbuf-size=__SIZE__+
6129 Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
6130 either for each Unix user (default), or for each instrumented process.
6132 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6134 |+--num-subbuf=__COUNT__+
6137 Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
6138 for each Unix user (default), or for each instrumented process.
6140 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6142 |+--tracefile-size=__SIZE__+
6145 Set the maximum size of each trace file that this channel writes within
6146 a stream to +__SIZE__+ bytes instead of no maximum.
6148 See <<tracefile-rotation,Trace file count and size>>.
6150 |+--tracefile-count=__COUNT__+
6153 Limit the number of trace files that this channel creates to
6154 +__COUNT__+ channels instead of no limit.
6156 See <<tracefile-rotation,Trace file count and size>>.
6158 |+--switch-timer=__PERIODUS__+
6161 Set the <<channel-switch-timer,switch timer period>>
6162 to +__PERIODUS__+{nbsp}µs.
6164 |+--read-timer=__PERIODUS__+
6167 Set the <<channel-read-timer,read timer period>>
6168 to +__PERIODUS__+{nbsp}µs.
6170 |[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+
6173 Set the timeout of user space applications which load LTTng-UST
6174 in blocking mode to +__TIMEOUTUS__+:
6177 Never block (non-blocking mode).
6180 Block forever until space is available in a sub-buffer to record
6183 __n__, a positive value::
6184 Wait for at most __n__ µs when trying to write into a sub-buffer.
6186 Note that, for this option to have any effect on an instrumented
6187 user space application, you need to run the application with a set
6188 env:LTTNG_UST_ALLOW_BLOCKING environment variable.
6190 |+--output=__TYPE__+ (Linux kernel tracing domain only)
6193 Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.
6197 You can only create a channel in the Linux kernel and user space
6198 <<domain,tracing domains>>: other tracing domains have their own channel
6199 created on the fly when <<enabling-disabling-events,creating event
6204 Because of a current LTTng limitation, you must create all channels
6205 _before_ you <<basic-tracing-session-control,start tracing>> in a given
6206 tracing session, that is, before the first time you run
6209 Since LTTng automatically creates a default channel when you use the
6210 man:lttng-enable-event(1) command with a specific tracing domain, you
6211 cannot, for example, create a Linux kernel event rule, start tracing,
6212 and then create a user space event rule, because no user space channel
6213 exists yet and it's too late to create one.
6215 For this reason, make sure to configure your channels properly
6216 before starting the tracers for the first time!
6219 The following examples show how you can combine the previous
6220 command-line options to create simple to more complex channels.
6222 .Create a Linux kernel channel with default attributes.
6226 $ lttng enable-channel --kernel my-channel
6230 .Create a user space channel with 4 sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
6234 $ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
6235 --buffers-pid my-channel
6239 .[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout.
6241 <<creating-destroying-tracing-sessions,Create a tracing-session>>,
6242 create the channel, <<enabling-disabling-events,create an event rule>>,
6243 and <<basic-tracing-session-control,start tracing>>:
6248 $ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
6249 $ lttng enable-event --userspace --channel=blocking-channel --all
6253 Run an application instrumented with LTTng-UST and allow it to block:
6257 $ LTTNG_UST_ALLOW_BLOCKING=1 my-app
6261 .Create a Linux kernel channel which rotates 8 trace files of 4{nbsp}MiB each for each stream
6265 $ lttng enable-channel --kernel --tracefile-count=8 \
6266 --tracefile-size=4194304 my-channel
6270 .Create a user space channel in overwrite (or _flight recorder_) mode.
6274 $ lttng enable-channel --userspace --overwrite my-channel
6278 You can <<enabling-disabling-events,create>> the same event rule in
6279 two different channels:
6283 $ lttng enable-event --userspace --channel=my-channel app:tp
6284 $ lttng enable-event --userspace --channel=other-channel app:tp
6287 If both channels are enabled, when a tracepoint named `app:tp` is
6288 reached, LTTng records two events, one for each channel.
6292 === Disable a channel
6294 To disable a specific channel that you <<enabling-disabling-channels,created>>
6295 previously, use the man:lttng-disable-channel(1) command.
6297 .Disable a specific Linux kernel channel.
6301 $ lttng disable-channel --kernel my-channel
6305 The state of a channel precedes the individual states of event rules
6306 attached to it: event rules which belong to a disabled channel, even if
6307 they are enabled, are also considered disabled.
6311 === Add context fields to a channel
6313 Event record fields in trace files provide important information about
6314 events that occured previously, but sometimes some external context may
6315 help you solve a problem faster. Examples of context fields are:
6317 * The **process ID**, **thread ID**, **process name**, and
6318 **process priority** of the thread in which the event occurs.
6319 * The **hostname** of the system on which the event occurs.
6320 * The current values of many possible **performance counters** using
6322 ** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
6324 ** Branch instructions, misses, and loads.
6326 * Any context defined at the application level (supported for the
6327 JUL and log4j <<domain,tracing domains>>).
6329 To get the full list of available context fields, see
6330 `lttng add-context --list`. Some context fields are reserved for a
6331 specific <<domain,tracing domain>> (Linux kernel or user space).
6333 You add context fields to <<channel,channels>>. All the events
6334 that a channel with added context fields records contain those fields.
6336 To add context fields to one or all the channels of a given tracing
6339 * Use the man:lttng-add-context(1) command.
6341 .Add context fields to all the channels of the current tracing session.
6343 The following command line adds the virtual process identifier and
6344 the per-thread CPU cycles count fields to all the user space channels
6345 of the current tracing session.
6349 $ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
6353 .Add performance counter context fields by raw ID
6355 See man:lttng-add-context(1) for the exact format of the context field
6356 type, which is partly compatible with the format used in
6361 $ lttng add-context --userspace --type=perf:thread:raw:r0110:test
6362 $ lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
6366 .Add a context field to a specific channel.
6368 The following command line adds the thread identifier context field
6369 to the Linux kernel channel named `my-channel` in the current
6374 $ lttng add-context --kernel --channel=my-channel --type=tid
6378 .Add an application-specific context field to a specific channel.
6380 The following command line adds the `cur_msg_id` context field of the
6381 `retriever` context retriever for all the instrumented
6382 <<java-application,Java applications>> recording <<event,event records>>
6383 in the channel named `my-channel`:
6387 $ lttng add-context --kernel --channel=my-channel \
6388 --type='$app:retriever:cur_msg_id'
6391 IMPORTANT: Make sure to always quote the `$` character when you
6392 use man:lttng-add-context(1) from a shell.
6395 NOTE: You cannot remove context fields from a channel once you add it.
6400 === Track process IDs
6402 It's often useful to allow only specific process IDs (PIDs) to emit
6403 events. For example, you may wish to record all the system calls made by
6404 a given process (Ă la http://linux.die.net/man/1/strace[strace]).
6406 The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
6407 purpose. Both commands operate on a whitelist of process IDs. You _add_
6408 entries to this whitelist with the man:lttng-track(1) command and remove
6409 entries with the man:lttng-untrack(1) command. Any process which has one
6410 of the PIDs in the whitelist is allowed to emit LTTng events which pass
6411 an enabled <<event,event rule>>.
6413 NOTE: The PID tracker tracks the _numeric process IDs_. Should a
6414 process with a given tracked ID exit and another process be given this
6415 ID, then the latter would also be allowed to emit events.
6417 .Track and untrack process IDs.
6419 For the sake of the following example, assume the target system has 16
6423 <<creating-destroying-tracing-sessions,create a tracing session>>,
6424 the whitelist contains all the possible PIDs:
6427 .All PIDs are tracked.
6428 image::track-all.png[]
6430 When the whitelist is full and you use the man:lttng-track(1) command to
6431 specify some PIDs to track, LTTng first clears the whitelist, then it
6432 tracks the specific PIDs. After:
6436 $ lttng track --pid=3,4,7,10,13
6442 .PIDs 3, 4, 7, 10, and 13 are tracked.
6443 image::track-3-4-7-10-13.png[]
6445 You can add more PIDs to the whitelist afterwards:
6449 $ lttng track --pid=1,15,16
6455 .PIDs 1, 15, and 16 are added to the whitelist.
6456 image::track-1-3-4-7-10-13-15-16.png[]
6458 The man:lttng-untrack(1) command removes entries from the PID tracker's
6459 whitelist. Given the previous example, the following command:
6463 $ lttng untrack --pid=3,7,10,13
6466 leads to this whitelist:
6469 .PIDs 3, 7, 10, and 13 are removed from the whitelist.
6470 image::track-1-4-15-16.png[]
6472 LTTng can track all possible PIDs again using the
6473 opt:lttng-track(1):--all option:
6477 $ lttng track --pid --all
6480 The result is, again:
6483 .All PIDs are tracked.
6484 image::track-all.png[]
6487 .Track only specific PIDs
6489 A very typical use case with PID tracking is to start with an empty
6490 whitelist, then <<basic-tracing-session-control,start the tracers>>, and
6491 then add PIDs manually while tracers are active. You can accomplish this
6492 by using the opt:lttng-untrack(1):--all option of the
6493 man:lttng-untrack(1) command to clear the whitelist after you
6494 <<creating-destroying-tracing-sessions,create a tracing session>>:
6498 $ lttng untrack --pid --all
6504 .No PIDs are tracked.
6505 image::untrack-all.png[]
6507 If you trace with this whitelist configuration, the tracer records no
6508 events for this <<domain,tracing domain>> because no processes are
6509 tracked. You can use the man:lttng-track(1) command as usual to track
6510 specific PIDs, for example:
6514 $ lttng track --pid=6,11
6520 .PIDs 6 and 11 are tracked.
6521 image::track-6-11.png[]
6526 [[saving-loading-tracing-session]]
6527 === Save and load tracing session configurations
6529 Configuring a <<tracing-session,tracing session>> can be long. Some of
6530 the tasks involved are:
6532 * <<enabling-disabling-channels,Create channels>> with
6533 specific attributes.
6534 * <<adding-context,Add context fields>> to specific channels.
6535 * <<enabling-disabling-events,Create event rules>> with specific log
6536 level and filter conditions.
6538 If you use LTTng to solve real world problems, chances are you have to
6539 record events using the same tracing session setup over and over,
6540 modifying a few variables each time in your instrumented program
6541 or environment. To avoid constant tracing session reconfiguration,
6542 the man:lttng(1) command-line tool can save and load tracing session
6543 configurations to/from XML files.
6545 To save a given tracing session configuration:
6547 * Use the man:lttng-save(1) command:
6552 $ lttng save my-session
6556 Replace `my-session` with the name of the tracing session to save.
6558 LTTng saves tracing session configurations to
6559 dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
6560 env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
6561 the opt:lttng-save(1):--output-path option to change this destination
6564 LTTng saves all configuration parameters, for example:
6566 * The tracing session name.
6567 * The trace data output path.
6568 * The channels with their state and all their attributes.
6569 * The context fields you added to channels.
6570 * The event rules with their state, log level and filter conditions.
6572 To load a tracing session:
6574 * Use the man:lttng-load(1) command:
6579 $ lttng load my-session
6583 Replace `my-session` with the name of the tracing session to load.
6585 When LTTng loads a configuration, it restores your saved tracing session
6586 as if you just configured it manually.
6588 See man:lttng(1) for the complete list of command-line options. You
6589 can also save and load all many sessions at a time, and decide in which
6590 directory to output the XML files.
6593 [[sending-trace-data-over-the-network]]
6594 === Send trace data over the network
6596 LTTng can send the recorded trace data to a remote system over the
6597 network instead of writing it to the local file system.
6599 To send the trace data over the network:
6601 . On the _remote_ system (which can also be the target system),
6602 start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
6611 . On the _target_ system, create a tracing session configured to
6612 send trace data over the network:
6617 $ lttng create my-session --set-url=net://remote-system
6621 Replace `remote-system` by the host name or IP address of the
6622 remote system. See man:lttng-create(1) for the exact URL format.
6624 . On the target system, use the man:lttng(1) command-line tool as usual.
6625 When tracing is active, the target's consumer daemon sends sub-buffers
6626 to the relay daemon running on the remote system instead of flushing
6627 them to the local file system. The relay daemon writes the received
6628 packets to the local file system.
6630 The relay daemon writes trace files to
6631 +$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
6632 +__hostname__+ is the host name of the target system and +__session__+
6633 is the tracing session name. Note that the env:LTTNG_HOME environment
6634 variable defaults to `$HOME` if not set. Use the
6635 opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
6636 trace files to another base directory.
6641 === View events as LTTng emits them (noch:{LTTng} live)
6643 LTTng live is a network protocol implemented by the <<lttng-relayd,relay
6644 daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
6645 display events as LTTng emits them on the target system while tracing is
6648 The relay daemon creates a _tee_: it forwards the trace data to both
6649 the local file system and to connected live viewers:
6652 .The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
6657 . On the _target system_, create a <<tracing-session,tracing session>>
6663 $ lttng create my-session --live
6667 This spawns a local relay daemon.
6669 . Start the live viewer and configure it to connect to the relay
6670 daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
6675 $ babeltrace --input-format=lttng-live \
6676 net://localhost/host/hostname/my-session
6683 * `hostname` with the host name of the target system.
6684 * `my-session` with the name of the tracing session to view.
6687 . Configure the tracing session as usual with the man:lttng(1)
6688 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6690 You can list the available live tracing sessions with Babeltrace:
6694 $ babeltrace --input-format=lttng-live net://localhost
6697 You can start the relay daemon on another system. In this case, you need
6698 to specify the relay daemon's URL when you create the tracing session
6699 with the opt:lttng-create(1):--set-url option. You also need to replace
6700 `localhost` in the procedure above with the host name of the system on
6701 which the relay daemon is running.
6703 See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
6704 command-line options.
6708 [[taking-a-snapshot]]
6709 === Take a snapshot of the current sub-buffers of a tracing session
6711 The normal behavior of LTTng is to append full sub-buffers to growing
6712 trace data files. This is ideal to keep a full history of the events
6713 that occurred on the target system, but it can
6714 represent too much data in some situations. For example, you may wish
6715 to trace your application continuously until some critical situation
6716 happens, in which case you only need the latest few recorded
6717 events to perform the desired analysis, not multi-gigabyte trace files.
6719 With the man:lttng-snapshot(1) command, you can take a snapshot of the
6720 current sub-buffers of a given <<tracing-session,tracing session>>.
6721 LTTng can write the snapshot to the local file system or send it over
6726 . Create a tracing session in _snapshot mode_:
6731 $ lttng create my-session --snapshot
6735 The <<channel-overwrite-mode-vs-discard-mode,event loss mode>> of
6736 <<channel,channels>> created in this mode is automatically set to
6737 _overwrite_ (flight recorder mode).
6739 . Configure the tracing session as usual with the man:lttng(1)
6740 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6742 . **Optional**: When you need to take a snapshot,
6743 <<basic-tracing-session-control,stop tracing>>.
6745 You can take a snapshot when the tracers are active, but if you stop
6746 them first, you are sure that the data in the sub-buffers does not
6747 change before you actually take the snapshot.
6754 $ lttng snapshot record --name=my-first-snapshot
6758 LTTng writes the current sub-buffers of all the current tracing
6759 session's channels to trace files on the local file system. Those trace
6760 files have `my-first-snapshot` in their name.
6762 There is no difference between the format of a normal trace file and the
6763 format of a snapshot: viewers of LTTng traces also support LTTng
6766 By default, LTTng writes snapshot files to the path shown by
6767 `lttng snapshot list-output`. You can change this path or decide to send
6768 snapshots over the network using either:
6770 . An output path or URL that you specify when you create the
6772 . An snapshot output path or URL that you add using
6773 `lttng snapshot add-output`
6774 . An output path or URL that you provide directly to the
6775 `lttng snapshot record` command.
6777 Method 3 overrides method 2, which overrides method 1. When you
6778 specify a URL, a relay daemon must listen on a remote system (see
6779 <<sending-trace-data-over-the-network,Send trace data over the network>>).
6784 === Use the machine interface
6786 With any command of the man:lttng(1) command-line tool, you can set the
6787 opt:lttng(1):--mi option to `xml` (before the command name) to get an
6788 XML machine interface output, for example:
6792 $ lttng --mi=xml enable-event --kernel --syscall open
6795 A schema definition (XSD) is
6796 https://github.com/lttng/lttng-tools/blob/stable-2.10/src/common/mi-lttng-3.0.xsd[available]
6797 to ease the integration with external tools as much as possible.
6801 [[metadata-regenerate]]
6802 === Regenerate the metadata of an LTTng trace
6804 An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
6805 data stream files and a metadata file. This metadata file contains,
6806 amongst other things, information about the offset of the clock sources
6807 used to timestamp <<event,event records>> when tracing.
6809 If, once a <<tracing-session,tracing session>> is
6810 <<basic-tracing-session-control,started>>, a major
6811 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
6812 happens, the trace's clock offset also needs to be updated. You
6813 can use the `metadata` item of the man:lttng-regenerate(1) command
6816 The main use case of this command is to allow a system to boot with
6817 an incorrect wall time and trace it with LTTng before its wall time
6818 is corrected. Once the system is known to be in a state where its
6819 wall time is correct, it can run `lttng regenerate metadata`.
6821 To regenerate the metadata of an LTTng trace:
6823 * Use the `metadata` item of the man:lttng-regenerate(1) command:
6828 $ lttng regenerate metadata
6834 `lttng regenerate metadata` has the following limitations:
6836 * Tracing session <<creating-destroying-tracing-sessions,created>>
6838 * User space <<channel,channels>>, if any, are using
6839 <<channel-buffering-schemes,per-user buffering>>.
6844 [[regenerate-statedump]]
6845 === Regenerate the state dump of a tracing session
6847 The LTTng kernel and user space tracers generate state dump
6848 <<event,event records>> when the application starts or when you
6849 <<basic-tracing-session-control,start a tracing session>>. An analysis
6850 can use the state dump event records to set an initial state before it
6851 builds the rest of the state from the following event records.
6852 http://tracecompass.org/[Trace Compass] is a notable example of an
6853 application which uses the state dump of an LTTng trace.
6855 When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
6856 state dump event records are not included in the snapshot because they
6857 were recorded to a sub-buffer that has been consumed or overwritten
6860 You can use the `lttng regenerate statedump` command to emit the state
6861 dump event records again.
6863 To regenerate the state dump of the current tracing session, provided
6864 create it in snapshot mode, before you take a snapshot:
6866 . Use the `statedump` item of the man:lttng-regenerate(1) command:
6871 $ lttng regenerate statedump
6875 . <<basic-tracing-session-control,Stop the tracing session>>:
6884 . <<taking-a-snapshot,Take a snapshot>>:
6889 $ lttng snapshot record --name=my-snapshot
6893 Depending on the event throughput, you should run steps 1 and 2
6894 as closely as possible.
6896 NOTE: To record the state dump events, you need to
6897 <<enabling-disabling-events,create event rules>> which enable them.
6898 LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`.
6899 LTTng-modules state dump tracepoints start with `lttng_statedump_`.
6903 [[persistent-memory-file-systems]]
6904 === Record trace data on persistent memory file systems
6906 https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
6907 (NVRAM) is random-access memory that retains its information when power
6908 is turned off (non-volatile). Systems with such memory can store data
6909 structures in RAM and retrieve them after a reboot, without flushing
6910 to typical _storage_.
6912 Linux supports NVRAM file systems thanks to either
6913 http://pramfs.sourceforge.net/[PRAMFS] or
6914 https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
6915 (requires Linux 4.1+).
6917 This section does not describe how to operate such file systems;
6918 we assume that you have a working persistent memory file system.
6920 When you create a <<tracing-session,tracing session>>, you can specify
6921 the path of the shared memory holding the sub-buffers. If you specify a
6922 location on an NVRAM file system, then you can retrieve the latest
6923 recorded trace data when the system reboots after a crash.
6925 To record trace data on a persistent memory file system and retrieve the
6926 trace data after a system crash:
6928 . Create a tracing session with a sub-buffer shared memory path located
6929 on an NVRAM file system:
6934 $ lttng create my-session --shm-path=/path/to/shm
6938 . Configure the tracing session as usual with the man:lttng(1)
6939 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6941 . After a system crash, use the man:lttng-crash(1) command-line tool to
6942 view the trace data recorded on the NVRAM file system:
6947 $ lttng-crash /path/to/shm
6951 The binary layout of the ring buffer files is not exactly the same as
6952 the trace files layout. This is why you need to use man:lttng-crash(1)
6953 instead of your preferred trace viewer directly.
6955 To convert the ring buffer files to LTTng trace files:
6957 * Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
6962 $ lttng-crash --extract=/path/to/trace /path/to/shm
6968 [[notif-trigger-api]]
6969 === Get notified when a channel's buffer usage is too high or too low
6971 With LTTng's $$C/C++$$ notification and trigger API, your user
6972 application can get notified when the buffer usage of one or more
6973 <<channel,channels>> becomes too low or too high. You can use this API
6974 and enable or disable <<event,event rules>> during tracing to avoid
6975 <<channel-overwrite-mode-vs-discard-mode,discarded event records>>.
6977 .Have a user application get notified when an LTTng channel's buffer usage is too high.
6979 In this example, we create and build an application which gets notified
6980 when the buffer usage of a specific LTTng channel is higher than
6981 75{nbsp}%. We only print that it is the case in the example, but we
6982 could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
6983 disable event rules when this happens.
6985 . Create the application's C source file:
6993 #include <lttng/domain.h>
6994 #include <lttng/action/action.h>
6995 #include <lttng/action/notify.h>
6996 #include <lttng/condition/condition.h>
6997 #include <lttng/condition/buffer-usage.h>
6998 #include <lttng/condition/evaluation.h>
6999 #include <lttng/notification/channel.h>
7000 #include <lttng/notification/notification.h>
7001 #include <lttng/trigger/trigger.h>
7002 #include <lttng/endpoint.h>
7004 int main(int argc, char *argv[])
7006 int exit_status = 0;
7007 struct lttng_notification_channel *notification_channel;
7008 struct lttng_condition *condition;
7009 struct lttng_action *action;
7010 struct lttng_trigger *trigger;
7011 const char *tracing_session_name;
7012 const char *channel_name;
7015 tracing_session_name = argv[1];
7016 channel_name = argv[2];
7019 * Create a notification channel. A notification channel
7020 * connects the user application to the LTTng session daemon.
7021 * This notification channel can be used to listen to various
7022 * types of notifications.
7024 notification_channel = lttng_notification_channel_create(
7025 lttng_session_daemon_notification_endpoint);
7028 * Create a "high buffer usage" condition. In this case, the
7029 * condition is reached when the buffer usage is greater than or
7030 * equal to 75 %. We create the condition for a specific tracing
7031 * session name, channel name, and for the user space tracing
7034 * The "low buffer usage" condition type also exists.
7036 condition = lttng_condition_buffer_usage_high_create();
7037 lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
7038 lttng_condition_buffer_usage_set_session_name(
7039 condition, tracing_session_name);
7040 lttng_condition_buffer_usage_set_channel_name(condition,
7042 lttng_condition_buffer_usage_set_domain_type(condition,
7046 * Create an action (get a notification) to take when the
7047 * condition created above is reached.
7049 action = lttng_action_notify_create();
7052 * Create a trigger. A trigger associates a condition to an
7053 * action: the action is executed when the condition is reached.
7055 trigger = lttng_trigger_create(condition, action);
7057 /* Register the trigger to LTTng. */
7058 lttng_register_trigger(trigger);
7061 * Now that we have registered a trigger, a notification will be
7062 * emitted everytime its condition is met. To receive this
7063 * notification, we must subscribe to notifications that match
7064 * the same condition.
7066 lttng_notification_channel_subscribe(notification_channel,
7070 * Notification loop. You can put this in a dedicated thread to
7071 * avoid blocking the main thread.
7074 struct lttng_notification *notification;
7075 enum lttng_notification_channel_status status;
7076 const struct lttng_evaluation *notification_evaluation;
7077 const struct lttng_condition *notification_condition;
7078 double buffer_usage;
7080 /* Receive the next notification. */
7081 status = lttng_notification_channel_get_next_notification(
7082 notification_channel, ¬ification);
7085 case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
7087 case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
7089 * The session daemon can drop notifications if
7090 * a monitoring application is not consuming the
7091 * notifications fast enough.
7094 case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
7096 * The notification channel has been closed by the
7097 * session daemon. This is typically caused by a session
7098 * daemon shutting down.
7102 /* Unhandled conditions or errors. */
7108 * A notification provides, amongst other things:
7110 * * The condition that caused this notification to be
7112 * * The condition evaluation, which provides more
7113 * specific information on the evaluation of the
7116 * The condition evaluation provides the buffer usage
7117 * value at the moment the condition was reached.
7119 notification_condition = lttng_notification_get_condition(
7121 notification_evaluation = lttng_notification_get_evaluation(
7124 /* We're subscribed to only one condition. */
7125 assert(lttng_condition_get_type(notification_condition) ==
7126 LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
7129 * Get the exact sampled buffer usage from the
7130 * condition evaluation.
7132 lttng_evaluation_buffer_usage_get_usage_ratio(
7133 notification_evaluation, &buffer_usage);
7136 * At this point, instead of printing a message, we
7137 * could do something to reduce the channel's buffer
7138 * usage, like disable specific events.
7140 printf("Buffer usage is %f %% in tracing session \"%s\", "
7141 "user space channel \"%s\".\n", buffer_usage * 100,
7142 tracing_session_name, channel_name);
7143 lttng_notification_destroy(notification);
7147 lttng_action_destroy(action);
7148 lttng_condition_destroy(condition);
7149 lttng_trigger_destroy(trigger);
7150 lttng_notification_channel_destroy(notification_channel);
7156 . Build the `notif-app` application, linking it to `liblttng-ctl`:
7161 $ gcc -o notif-app notif-app.c -llttng-ctl
7165 . <<creating-destroying-tracing-sessions,Create a tracing session>>,
7166 <<enabling-disabling-events,create an event rule>> matching all the
7167 user space tracepoints, and
7168 <<basic-tracing-session-control,start tracing>>:
7173 $ lttng create my-session
7174 $ lttng enable-event --userspace --all
7179 If you create the channel manually with the man:lttng-enable-channel(1)
7180 command, you can control how frequently are the current values of the
7181 channel's properties sampled to evaluate user conditions with the
7182 opt:lttng-enable-channel(1):--monitor-timer option.
7184 . Run the `notif-app` application. This program accepts the
7185 <<tracing-session,tracing session>> name and the user space channel
7186 name as its two first arguments. The channel which LTTng automatically
7187 creates with the man:lttng-enable-event(1) command above is named
7193 $ ./notif-app my-session channel0
7197 . In another terminal, run an application with a very high event
7198 throughput so that the 75{nbsp}% buffer usage condition is reached.
7200 In the first terminal, the application should print lines like this:
7203 Buffer usage is 81.45197 % in tracing session "my-session", user space
7207 If you don't see anything, try modifying the condition in
7208 path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
7209 (step 2) and running it again (step 4).
7216 [[lttng-modules-ref]]
7217 === noch:{LTTng-modules}
7221 [[lttng-tracepoint-enum]]
7222 ==== `LTTNG_TRACEPOINT_ENUM()` usage
7224 Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration:
7228 LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))
7233 * `name` with the name of the enumeration (C identifier, unique
7234 amongst all the defined enumerations).
7235 * `entries` with a list of enumeration entries.
7237 The available enumeration entry macros are:
7239 +ctf_enum_value(__name__, __value__)+::
7240 Entry named +__name__+ mapped to the integral value +__value__+.
7242 +ctf_enum_range(__name__, __begin__, __end__)+::
7243 Entry named +__name__+ mapped to the range of integral values between
7244 +__begin__+ (included) and +__end__+ (included).
7246 +ctf_enum_auto(__name__)+::
7247 Entry named +__name__+ mapped to the integral value following the
7248 last mapping's value.
7250 The last value of a `ctf_enum_value()` entry is its +__value__+
7253 The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
7255 If `ctf_enum_auto()` is the first entry in the list, its integral
7258 Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
7259 to use a defined enumeration as a tracepoint field.
7261 .Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
7265 LTTNG_TRACEPOINT_ENUM(
7268 ctf_enum_auto("AUTO: EXPECT 0")
7269 ctf_enum_value("VALUE: 23", 23)
7270 ctf_enum_value("VALUE: 27", 27)
7271 ctf_enum_auto("AUTO: EXPECT 28")
7272 ctf_enum_range("RANGE: 101 TO 303", 101, 303)
7273 ctf_enum_auto("AUTO: EXPECT 304")
7281 [[lttng-modules-tp-fields]]
7282 ==== Tracepoint fields macros (for `TP_FIELDS()`)
7284 [[tp-fast-assign]][[tp-struct-entry]]The available macros to define
7285 tracepoint fields, which must be listed within `TP_FIELDS()` in
7286 `LTTNG_TRACEPOINT_EVENT()`, are:
7288 [role="func-desc growable",cols="asciidoc,asciidoc"]
7289 .Available macros to define LTTng-modules tracepoint fields
7291 |Macro |Description and parameters
7294 +ctf_integer(__t__, __n__, __e__)+
7296 +ctf_integer_nowrite(__t__, __n__, __e__)+
7298 +ctf_user_integer(__t__, __n__, __e__)+
7300 +ctf_user_integer_nowrite(__t__, __n__, __e__)+
7302 Standard integer, displayed in base 10.
7305 Integer C type (`int`, `long`, `size_t`, ...).
7311 Argument expression.
7314 +ctf_integer_hex(__t__, __n__, __e__)+
7316 +ctf_user_integer_hex(__t__, __n__, __e__)+
7318 Standard integer, displayed in base 16.
7327 Argument expression.
7329 |+ctf_integer_oct(__t__, __n__, __e__)+
7331 Standard integer, displayed in base 8.
7340 Argument expression.
7343 +ctf_integer_network(__t__, __n__, __e__)+
7345 +ctf_user_integer_network(__t__, __n__, __e__)+
7347 Integer in network byte order (big-endian), displayed in base 10.
7356 Argument expression.
7359 +ctf_integer_network_hex(__t__, __n__, __e__)+
7361 +ctf_user_integer_network_hex(__t__, __n__, __e__)+
7363 Integer in network byte order, displayed in base 16.
7372 Argument expression.
7375 +ctf_enum(__N__, __t__, __n__, __e__)+
7377 +ctf_enum_nowrite(__N__, __t__, __n__, __e__)+
7379 +ctf_user_enum(__N__, __t__, __n__, __e__)+
7381 +ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+
7386 Name of a <<lttng-tracepoint-enum,previously defined enumeration>>.
7389 Integer C type (`int`, `long`, `size_t`, ...).
7395 Argument expression.
7398 +ctf_string(__n__, __e__)+
7400 +ctf_string_nowrite(__n__, __e__)+
7402 +ctf_user_string(__n__, __e__)+
7404 +ctf_user_string_nowrite(__n__, __e__)+
7406 Null-terminated string; undefined behavior if +__e__+ is `NULL`.
7412 Argument expression.
7415 +ctf_array(__t__, __n__, __e__, __s__)+
7417 +ctf_array_nowrite(__t__, __n__, __e__, __s__)+
7419 +ctf_user_array(__t__, __n__, __e__, __s__)+
7421 +ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
7423 Statically-sized array of integers.
7426 Array element C type.
7432 Argument expression.
7438 +ctf_array_bitfield(__t__, __n__, __e__, __s__)+
7440 +ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7442 +ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
7444 +ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7446 Statically-sized array of bits.
7448 The type of +__e__+ must be an integer type. +__s__+ is the number
7449 of elements of such type in +__e__+, not the number of bits.
7452 Array element C type.
7458 Argument expression.
7464 +ctf_array_text(__t__, __n__, __e__, __s__)+
7466 +ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
7468 +ctf_user_array_text(__t__, __n__, __e__, __s__)+
7470 +ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
7472 Statically-sized array, printed as text.
7474 The string does not need to be null-terminated.
7477 Array element C type (always `char`).
7483 Argument expression.
7489 +ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
7491 +ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7493 +ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
7495 +ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7497 Dynamically-sized array of integers.
7499 The type of +__E__+ must be unsigned.
7502 Array element C type.
7508 Argument expression.
7511 Length expression C type.
7517 +ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7519 +ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7521 Dynamically-sized array of integers, displayed in base 16.
7523 The type of +__E__+ must be unsigned.
7526 Array element C type.
7532 Argument expression.
7535 Length expression C type.
7540 |+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
7542 Dynamically-sized array of integers in network byte order (big-endian),
7543 displayed in base 10.
7545 The type of +__E__+ must be unsigned.
7548 Array element C type.
7554 Argument expression.
7557 Length expression C type.
7563 +ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7565 +ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7567 +ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7569 +ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7571 Dynamically-sized array of bits.
7573 The type of +__e__+ must be an integer type. +__s__+ is the number
7574 of elements of such type in +__e__+, not the number of bits.
7576 The type of +__E__+ must be unsigned.
7579 Array element C type.
7585 Argument expression.
7588 Length expression C type.
7594 +ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7596 +ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7598 +ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7600 +ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7602 Dynamically-sized array, displayed as text.
7604 The string does not need to be null-terminated.
7606 The type of +__E__+ must be unsigned.
7608 The behaviour is undefined if +__e__+ is `NULL`.
7611 Sequence element C type (always `char`).
7617 Argument expression.
7620 Length expression C type.
7626 Use the `_user` versions when the argument expression, `e`, is
7627 a user space address. In the cases of `ctf_user_integer*()` and
7628 `ctf_user_float*()`, `&e` must be a user space address, thus `e` must
7631 The `_nowrite` versions omit themselves from the session trace, but are
7632 otherwise identical. This means the `_nowrite` fields won't be written
7633 in the recorded trace. Their primary purpose is to make some
7634 of the event context available to the
7635 <<enabling-disabling-events,event filters>> without having to
7636 commit the data to sub-buffers.
7642 Terms related to LTTng and to tracing in general:
7645 The http://diamon.org/babeltrace[Babeltrace] project, which includes
7646 the cmd:babeltrace command, some libraries, and Python bindings.
7648 <<channel-buffering-schemes,buffering scheme>>::
7649 A layout of sub-buffers applied to a given channel.
7651 <<channel,channel>>::
7652 An entity which is responsible for a set of ring buffers.
7654 <<event,Event rules>> are always attached to a specific channel.
7657 A reference of time for a tracer.
7659 <<lttng-consumerd,consumer daemon>>::
7660 A process which is responsible for consuming the full sub-buffers
7661 and write them to a file system or send them over the network.
7663 <<channel-overwrite-mode-vs-discard-mode,discard mode>>:: The event loss
7664 mode in which the tracer _discards_ new event records when there's no
7665 sub-buffer space left to store them.
7668 The consequence of the execution of an instrumentation
7669 point, like a tracepoint that you manually place in some source code,
7670 or a Linux kernel KProbe.
7672 An event is said to _occur_ at a specific time. Different actions can
7673 be taken upon the occurrence of an event, like record the event's payload
7676 <<channel-overwrite-mode-vs-discard-mode,event loss mode>>::
7677 The mechanism by which event records of a given channel are lost
7678 (not recorded) when there is no sub-buffer space left to store them.
7680 [[def-event-name]]event name::
7681 The name of an event, which is also the name of the event record.
7682 This is also called the _instrumentation point name_.
7685 A record, in a trace, of the payload of an event which occured.
7687 <<event,event rule>>::
7688 Set of conditions which must be satisfied for one or more occuring
7689 events to be recorded.
7691 `java.util.logging`::
7693 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].
7695 <<instrumenting,instrumentation>>::
7696 The use of LTTng probes to make a piece of software traceable.
7698 instrumentation point::
7699 A point in the execution path of a piece of software that, when
7700 reached by this execution, can emit an event.
7702 instrumentation point name::
7703 See _<<def-event-name,event name>>_.
7706 A http://logging.apache.org/log4j/1.2/[logging library] for Java
7707 developed by the Apache Software Foundation.
7710 Level of severity of a log statement or user space
7711 instrumentation point.
7714 The _Linux Trace Toolkit: next generation_ project.
7716 <<lttng-cli,cmd:lttng>>::
7717 A command-line tool provided by the LTTng-tools project which you
7718 can use to send and receive control messages to and from a
7722 The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
7723 which is a set of analyzing programs that are used to obtain a
7724 higher level view of an LTTng trace.
7726 cmd:lttng-consumerd::
7727 The name of the consumer daemon program.
7730 A utility provided by the LTTng-tools project which can convert
7731 ring buffer files (usually
7732 <<persistent-memory-file-systems,saved on a persistent memory file system>>)
7735 LTTng Documentation::
7738 <<lttng-live,LTTng live>>::
7739 A communication protocol between the relay daemon and live viewers
7740 which makes it possible to see events "live", as they are received by
7743 <<lttng-modules,LTTng-modules>>::
7744 The https://github.com/lttng/lttng-modules[LTTng-modules] project,
7745 which contains the Linux kernel modules to make the Linux kernel
7746 instrumentation points available for LTTng tracing.
7749 The name of the relay daemon program.
7751 cmd:lttng-sessiond::
7752 The name of the session daemon program.
7755 The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
7756 contains the various programs and libraries used to
7757 <<controlling-tracing,control tracing>>.
7759 <<lttng-ust,LTTng-UST>>::
7760 The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
7761 contains libraries to instrument user applications.
7763 <<lttng-ust-agents,LTTng-UST Java agent>>::
7764 A Java package provided by the LTTng-UST project to allow the
7765 LTTng instrumentation of `java.util.logging` and Apache log4j 1.2
7768 <<lttng-ust-agents,LTTng-UST Python agent>>::
7769 A Python package provided by the LTTng-UST project to allow the
7770 LTTng instrumentation of Python logging statements.
7772 <<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
7773 The event loss mode in which new event records overwrite older
7774 event records when there's no sub-buffer space left to store them.
7776 <<channel-buffering-schemes,per-process buffering>>::
7777 A buffering scheme in which each instrumented process has its own
7778 sub-buffers for a given user space channel.
7780 <<channel-buffering-schemes,per-user buffering>>::
7781 A buffering scheme in which all the processes of a Unix user share the
7782 same sub-buffer for a given user space channel.
7784 <<lttng-relayd,relay daemon>>::
7785 A process which is responsible for receiving the trace data sent by
7786 a distant consumer daemon.
7789 A set of sub-buffers.
7791 <<lttng-sessiond,session daemon>>::
7792 A process which receives control commands from you and orchestrates
7793 the tracers and various LTTng daemons.
7795 <<taking-a-snapshot,snapshot>>::
7796 A copy of the current data of all the sub-buffers of a given tracing
7797 session, saved as trace files.
7800 One part of an LTTng ring buffer which contains event records.
7803 The time information attached to an event when it is emitted.
7806 A set of files which are the concatenations of one or more
7807 flushed sub-buffers.
7810 The action of recording the events emitted by an application
7811 or by a system, or to initiate such recording by controlling
7815 The http://tracecompass.org[Trace Compass] project and application.
7818 An instrumentation point using the tracepoint mechanism of the Linux
7819 kernel or of LTTng-UST.
7821 tracepoint definition::
7822 The definition of a single tracepoint.
7825 The name of a tracepoint.
7827 tracepoint provider::
7828 A set of functions providing tracepoints to an instrumented user
7831 Not to be confused with a _tracepoint provider package_: many tracepoint
7832 providers can exist within a tracepoint provider package.
7834 tracepoint provider package::
7835 One or more tracepoint providers compiled as an object file or as
7839 A software which records emitted events.
7841 <<domain,tracing domain>>::
7842 A namespace for event sources.
7844 <<tracing-group,tracing group>>::
7845 The Unix group in which a Unix user can be to be allowed to trace the
7848 <<tracing-session,tracing session>>::
7849 A stateful dialogue between you and a <<lttng-sessiond,session
7853 An application running in user space, as opposed to a Linux kernel
7854 module, for example.