2 id: viewing-and-analyzing-your-traces
5 This section describes how to visualize the data gathered after tracing
6 the Linux kernel or a user space application.
8 Many ways exist to read your LTTng traces:
10 * **`babeltrace`** is a command line utility which converts trace formats;
11 it supports the format used by LTTng,
12 <abbr title="Common Trace Format">CTF</abbr>, as well as a basic
13 text output which may be `grep`ed. The `babeltrace` command is
15 <a href="http://www.efficios.com/babeltrace" class="ext">Babeltrace</a> project.
16 * Babeltrace also includes a **Python binding** so that you may
17 easily open and read an LTTng trace with your own script, benefiting
18 from the power of Python.
19 * **<a href="http://projects.eclipse.org/projects/tools.tracecompass" class="ext">Trace Compass</a>**
20 is an Eclipse plugin used to visualize and analyze various types of
21 traces, including LTTng's. It also comes as a standalone application
22 and can be downloaded from
23 <a href="http://secretaire.dorsal.polymtl.ca/~gbastien/TracingRCP/TraceCompass/" class="ext">here</a>
24 for a daily build of the latest source code. A version containing
25 some experimental features like Virtual Machine analysis and
26 Critical Path analysis is also available
27 <a href="http://secretaire.dorsal.polymtl.ca/~gbastien/TracingRCP/DorsalExperimental/" class="ext">here</a>.
29 LTTng trace files are usually recorded in the `~/lttng-traces` directory.
30 Let's now view the trace and perform a basic analysis using
33 The simplest way to list all the recorded events of a trace is to pass its
34 path to `babeltrace` with no options:
37 babeltrace ~/lttng-traces/my-session
40 `babeltrace` will find all traces within the given path recursively and
41 output all their events, merging them intelligently.
43 Listing all the system calls of a Linux kernel trace with their arguments is
44 easy with `babeltrace` and `grep`:
47 babeltrace ~/lttng-traces/my-kernel-session | grep sys_
50 Counting events is also straightforward:
53 babeltrace ~/lttng-traces/my-kernel-session | grep sys_read | wc --lines
56 The text output of `babeltrace` is useful for isolating events by simple
57 matching using `grep` and similar utilities. However, more elaborate filters
58 such as keeping only events with a field value falling within a specific range
59 are not trivial to write using a shell. Moreover, reductions and even the
60 most basic computations involving multiple events are virtually impossible
63 Fortunately, Babeltrace ships with a Python 3 binding which makes it
64 really easy to read the events of an LTTng trace sequentially and compute
65 the desired information.
67 Here's a simple example using the Babeltrace Python binding. The following
68 script accepts an LTTng Linux kernel trace path as its first argument and
69 outputs the short names of the top 5 running processes on CPU 0 during the
74 from collections import Counter
79 if len(sys.argv) != 2:
80 msg = 'Usage: python {} TRACEPATH'.format(sys.argv[0])
83 # a trace collection holds one to many traces
84 col = babeltrace.TraceCollection()
86 # add the trace provided by the user
87 # (LTTng traces always have the 'ctf' format)
88 if col.add_trace(sys.argv[1], 'ctf') is None:
89 raise RuntimeError('Cannot add trace')
91 # this counter dict will hold execution times:
93 # task command name -> total execution time (ns)
94 exec_times = Counter()
96 # this holds the last `sched_switch` timestamp
100 for event in col.events:
101 # keep only `sched_switch` events
102 if event.name != 'sched_switch':
105 # keep only events which happened on CPU 0
106 if event['cpu_id'] != 0:
110 cur_ts = event.timestamp
116 # previous task command (short) name
117 prev_comm = event['prev_comm']
119 # initialize entry in our dict if not yet done
120 if prev_comm not in exec_times:
121 exec_times[prev_comm] = 0
123 # compute previous command execution time
124 diff = cur_ts - last_ts
126 # update execution time of this command
127 exec_times[prev_comm] += diff
129 # update last timestamp
133 for name, ns in exec_times.most_common()[:5]:
135 print('{:20}{} s'.format(name, s))
138 if __name__ == '__main__':
142 Save this script as `top5proc.py` and run it with Python 3, providing the
143 path to an LTTng Linux kernel trace as the first argument:
146 python3 top5proc.py ~/lttng-sessions/my-session-.../kernel
149 Make sure the path you provide is the directory containing actual trace
150 files (`channel0_0`, `metadata`, etc.): the `babeltrace` utility recurses
151 directories, but the Python binding does not.
153 Here's an example of output:
156 swapper/0 48.607245889 s
157 chromium 7.192738188 s
158 pavucontrol 0.709894415 s
159 Compositor 0.660867933 s
160 Xorg.bin 0.616753786 s
163 Note that `swapper/0` is the "idle" process of CPU 0 on Linux; since we
164 weren't using the CPU that much when tracing, its first position in the list