Commit | Line | Data |
---|---|---|
b4867b3b PP |
1 | lttng-enable-channel(1) |
2 | ======================= | |
3 | ||
4 | ||
5 | NAME | |
6 | ---- | |
7 | lttng-enable-channel - Create or enable LTTng channels | |
8 | ||
9 | ||
10 | SYNOPSIS | |
11 | -------- | |
12 | Create a Linux kernel channel: | |
13 | ||
14 | [verse] | |
ce19b9ed | 15 | *lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *enable-channel* option:--kernel |
240311ba | 16 | [option:--overwrite] [option:--output=(`mmap` | `splice`)] |
b4867b3b PP |
17 | [option:--subbuf-size='SIZE'] [option:--num-subbuf='COUNT'] |
18 | [option:--switch-timer='PERIODUS'] [option:--read-timer='PERIODUS'] | |
961df0d0 | 19 | [option:--monitor-timer='PERIODUS'] |
b4867b3b PP |
20 | [option:--tracefile-size='SIZE'] [option:--tracefile-count='COUNT'] |
21 | [option:--session='SESSION'] 'CHANNEL' | |
22 | ||
23 | Create a user space channel: | |
24 | ||
25 | [verse] | |
ce19b9ed | 26 | *lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *enable-channel* option:--userspace |
240311ba | 27 | [option:--overwrite] [option:--buffers-pid] |
b4867b3b PP |
28 | [option:--subbuf-size='SIZE'] [option:--num-subbuf='COUNT'] |
29 | [option:--switch-timer='PERIODUS'] [option:--read-timer='PERIODUS'] | |
961df0d0 | 30 | [option:--monitor-timer='PERIODUS'] [option:--blocking-timeout='TIMEOUTUS'] |
b4867b3b PP |
31 | [option:--tracefile-size='SIZE'] [option:--tracefile-count='COUNT'] |
32 | [option:--session='SESSION'] 'CHANNEL' | |
33 | ||
34 | Enable existing channel(s): | |
35 | ||
36 | [verse] | |
ce19b9ed | 37 | *lttng* ['linkgenoptions:(GENERAL OPTIONS)'] *enable-channel* (option:--userspace | option:--kernel) |
b4867b3b PP |
38 | [option:--session='SESSION'] 'CHANNEL'[,'CHANNEL']... |
39 | ||
40 | ||
41 | DESCRIPTION | |
42 | ----------- | |
43 | The `lttng enable-channel` command can create a new channel, or enable | |
44 | one or more existing and disabled ones. | |
45 | ||
46 | A channel is the owner of sub-buffers holding recorded events. Event, | |
7c1a4458 | 47 | rules, when created using man:lttng-enable-event(1), are always |
b4867b3b PP |
48 | assigned to a channel. When creating a new channel, many parameters |
49 | related to those sub-buffers can be fine-tuned. They are described in | |
50 | the subsections below. | |
51 | ||
52 | When 'CHANNEL' does not name an existing channel, a channel named | |
53 | 'CHANNEL' is created. Otherwise, the disabled channel named 'CHANNEL' | |
54 | is enabled. | |
55 | ||
7c1a4458 | 56 | Note that the man:lttng-enable-event(1) command can automatically |
b4867b3b PP |
57 | create default channels when no channel exist. |
58 | ||
59 | A channel is always contained in a tracing session | |
7c1a4458 | 60 | (see man:lttng-create(1) for creating a tracing session). The |
b4867b3b PP |
61 | session in which a channel is created using `lttng enable-channel` can |
62 | be specified using the option:--session option. If the option:--session | |
63 | option is omitted, the current tracing session is targeted. | |
64 | ||
65 | Existing enabled channels can be disabled using | |
7c1a4458 PP |
66 | man:lttng-disable-channel(1). Channels of a given session can be |
67 | listed using man:lttng-list(1). | |
b4867b3b | 68 | |
1076f2b7 PP |
69 | See the <<limitations,LIMITATIONS>> section below for a list of |
70 | limitations of this command to consider. | |
b4867b3b PP |
71 | |
72 | ||
73 | Event loss modes | |
74 | ~~~~~~~~~~~~~~~~ | |
75 | LTTng tracers are non-blocking: when no empty sub-buffer exists, | |
76 | losing events is acceptable when the alternative would be to cause | |
77 | substantial delays in the instrumented application's execution. | |
78 | ||
79 | LTTng privileges performance over integrity, aiming at perturbing the | |
80 | traced system as little as possible in order to make tracing of subtle | |
81 | race conditions and rare interrupt cascades possible. | |
82 | ||
83 | When it comes to losing events because no empty sub-buffer is available, | |
84 | the channel's event loss mode, specified by one of the option:--discard | |
85 | and option:--overwrite options, determines what to do amongst: | |
86 | ||
87 | Discard:: | |
88 | Drop the newest events until a sub-buffer is released. | |
89 | ||
90 | Overwrite:: | |
91 | Clear the sub-buffer containing the oldest recorded events and start | |
92 | recording the newest events there. This mode is sometimes called | |
93 | _flight recorder mode_ because it behaves like a flight recorder: | |
94 | always keep a fixed amount of the latest data. | |
95 | ||
96 | Which mechanism to choose depends on the context: prioritize the newest | |
97 | or the oldest events in the ring buffer? | |
98 | ||
99 | Beware that, in overwrite mode (option:--overwrite option), a whole | |
100 | sub-buffer is abandoned as soon as a new event doesn't find an empty | |
101 | sub-buffer, whereas in discard mode (option:--discard option), only the | |
102 | event that doesn't fit is discarded. | |
103 | ||
104 | Also note that a count of lost events is incremented and saved in the | |
105 | trace itself when an event is lost in discard mode, whereas no | |
106 | information is kept when a sub-buffer gets overwritten before being | |
107 | committed. | |
108 | ||
109 | The probability of losing events, if it is experience in a given | |
110 | context, can be reduced by fine-tuning the sub-buffers count and size | |
111 | (see next subsection). | |
112 | ||
113 | ||
114 | Sub-buffers count and size | |
115 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
116 | The option:--num-subbuf and option:--subbuf-size options respectively | |
117 | set the number of sub-buffers and their individual size when creating | |
118 | a new channel. | |
119 | ||
120 | Note that there is a noticeable tracer's CPU overhead introduced when | |
121 | switching sub-buffers (marking a full one as consumable and switching | |
122 | to an empty one for the following events to be recorded). Knowing this, | |
123 | the following list presents a few practical situations along with how | |
124 | to configure sub-buffers for them when creating a channel in overwrite | |
125 | mode (option:--overwrite option): | |
126 | ||
127 | High event throughput:: | |
128 | In general, prefer bigger sub-buffers to lower the risk of losing | |
129 | events. Having bigger sub-buffers also ensures a lower sub-buffer | |
130 | switching frequency. The number of sub-buffers is only meaningful | |
131 | if the channel is enabled in overwrite mode: in this case, if a | |
132 | sub-buffer overwrite happens, the other sub-buffers | |
133 | are left unaltered. | |
134 | ||
135 | Low event throughput:: | |
136 | In general, prefer smaller sub-buffers since the risk of losing | |
137 | events is already low. Since events happen less frequently, the | |
138 | sub-buffer switching frequency should remain low and thus the | |
139 | tracer's overhead should not be a problem. | |
140 | ||
141 | Low memory system:: | |
142 | If the target system has a low memory limit, prefer fewer first, | |
143 | then smaller sub-buffers. Even if the system is limited in memory, | |
144 | it is recommended to keep the sub-buffers as big as possible to | |
145 | avoid a high sub-buffer switching frequency. | |
146 | ||
147 | In discard mode (option:--discard option), the sub-buffers count | |
148 | parameter is pointless: using two sub-buffers and setting their size | |
149 | according to the requirements of the context is fine. | |
150 | ||
151 | ||
961df0d0 PP |
152 | Switch timer |
153 | ~~~~~~~~~~~~ | |
b4867b3b PP |
154 | When a channel's switch timer fires, a sub-buffer switch happens. This |
155 | timer may be used to ensure that event data is consumed and committed | |
156 | to trace files periodically in case of a low event throughput. | |
157 | ||
158 | It's also convenient when big sub-buffers are used to cope with sporadic | |
159 | high event throughput, even if the throughput is normally lower. | |
160 | ||
961df0d0 PP |
161 | Use the option:--switch-timer option to control the switch timer's |
162 | period of the channel to create. | |
163 | ||
164 | ||
165 | Read timer | |
166 | ~~~~~~~~~~ | |
167 | By default, an internal notification mechanism is used to signal a full | |
168 | sub-buffer so that it can be consumed. When such notifications must be | |
169 | avoided, for example in real-time applications, the channel's read timer | |
170 | can be used instead. When the read timer fires, sub-buffers are checked | |
171 | for consumption when they are full. | |
172 | ||
173 | Use the option:--read-timer option to control the read timer's period of | |
174 | the channel to create. | |
175 | ||
176 | ||
177 | Monitor timer | |
178 | ~~~~~~~~~~~~~ | |
179 | When a channel's monitor timer fires, its registered trigger conditions | |
180 | are evaluated using the current values of its properties (for example, | |
181 | the current usage of its sub-buffers). When a trigger condition is true, | |
182 | LTTng executes its associated action. The only type of action currently | |
183 | supported is to notify one or more user applications. | |
184 | ||
185 | See the installed $$C/C++$$ headers in `lttng/action`, | |
186 | `lttng/condition`, `lttng/notification`, and `lttng/trigger` to learn | |
187 | more about application notifications and triggers. | |
188 | ||
189 | Use the option:--monitor-timer option to control the monitor timer's | |
190 | period of the channel to create. | |
b4867b3b PP |
191 | |
192 | ||
193 | Buffering scheme | |
194 | ~~~~~~~~~~~~~~~~ | |
195 | In the user space tracing domain, two buffering schemes are available | |
196 | when creating a channel: | |
197 | ||
198 | Per-process buffering (option:--buffers-pid option):: | |
199 | Keep one ring buffer per process. | |
200 | ||
201 | Per-user buffering (option:--buffers-uid option):: | |
202 | Keep one ring buffer for all the processes of a single user. | |
203 | ||
204 | The per-process buffering scheme consumes more memory than the per-user | |
205 | option if more than one process is instrumented for LTTng-UST. | |
206 | However, per-process buffering ensures that one process having a high | |
207 | event throughput won't fill all the shared sub-buffers, only its own. | |
208 | ||
209 | The Linux kernel tracing domain only has one available buffering scheme | |
210 | which is to use a single ring buffer for the whole system | |
211 | (option:--buffers-global option). | |
212 | ||
213 | ||
214 | Trace files limit and size | |
215 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
216 | By default, trace files can grow as large as needed. The maximum size | |
217 | of each trace file written by a channel can be set on creation using the | |
218 | option:--tracefile-size option. When such a trace file's size reaches | |
219 | the channel's fixed maximum size, another trace file is created to hold | |
220 | the next recorded events. A file count is appended to each trace file | |
221 | name in this case. | |
222 | ||
223 | If the option:--tracefile-size option is used, the maximum number of | |
224 | created trace files is unlimited. To limit them, the | |
225 | option:--tracefile-count option can be used. This option is always used | |
226 | in conjunction with the option:--tracefile-size option. | |
227 | ||
228 | For example, consider this command: | |
229 | ||
d4f093aa | 230 | [role="term"] |
03c5529d PP |
231 | ---- |
232 | $ lttng enable-channel --kernel --tracefile-size=4096 \ | |
b4867b3b | 233 | --tracefile-count=32 my-channel |
03c5529d | 234 | ---- |
b4867b3b PP |
235 | |
236 | Here, for each stream, the maximum size of each trace file is | |
237 | 4 kiB and there can be a maximum of 32 different files. When there is | |
238 | no space left in the last file, _trace file rotation_ happens: the first | |
239 | file is cleared and new sub-buffers containing events are written there. | |
240 | ||
241 | ||
242 | include::common-cmd-options-head.txt[] | |
243 | ||
244 | ||
245 | Domain | |
246 | ~~~~~~ | |
247 | One of: | |
248 | ||
249 | option:-k, option:--kernel:: | |
250 | Enable channel in the Linux kernel domain. | |
251 | ||
252 | option:-u, option:--userspace:: | |
253 | Enable channel in the user space domain. | |
254 | ||
255 | ||
256 | Target | |
257 | ~~~~~~ | |
59b19c3c | 258 | option:-s 'SESSION', option:--session='SESSION':: |
b4867b3b PP |
259 | Create or enable channel in the tracing session named 'SESSION' |
260 | instead of the current tracing session. | |
261 | ||
262 | ||
263 | Event loss mode | |
264 | ~~~~~~~~~~~~~~~ | |
265 | One of: | |
266 | ||
267 | option:--discard:: | |
268 | Discard events when sub-buffers are full (default). | |
269 | ||
270 | option:--overwrite:: | |
271 | Flight recorder mode: always keep a fixed amount of the latest | |
272 | data. | |
273 | ||
274 | ||
275 | Sub-buffers | |
276 | ~~~~~~~~~~~ | |
277 | option:--num-subbuf='COUNT':: | |
278 | Use 'COUNT' sub-buffers. Rounded up to the next power of two. | |
279 | + | |
280 | Default values: | |
281 | + | |
c93eadad PP |
282 | * option:--userspace and option:--buffers-uid options: |
283 | {default_ust_uid_channel_subbuf_num} | |
284 | * option:--userspace and option:--buffers-pid options: | |
285 | {default_ust_pid_channel_subbuf_num} | |
286 | * option:--kernel option: {default_kernel_channel_subbuf_num} | |
287 | * `metadata` channel: {default_metadata_subbuf_num} | |
b4867b3b | 288 | |
55d7bb02 PP |
289 | option:--output='TYPE':: |
290 | Set channel's output type to 'TYPE'. | |
291 | + | |
292 | Available types: `mmap` (always available) and `splice` (only available | |
293 | with the option:--kernel option). | |
294 | + | |
295 | Default values: | |
296 | + | |
297 | * option:--userspace and option:--buffers-uid options: `mmap` | |
298 | * option:--userspace and option:--buffers-pid options: `mmap` | |
299 | * option:--kernel option: `splice` | |
300 | * `metadata` channel: `mmap` | |
301 | ||
b4867b3b PP |
302 | option:--subbuf-size='SIZE':: |
303 | Set the individual size of sub-buffers to 'SIZE' bytes. | |
304 | The `k` (kiB), `M` (MiB), and `G` (GiB) suffixes are supported. | |
305 | Rounded up to the next power of two. | |
306 | + | |
307 | The minimum sub-buffer size, for each tracer, is the maximum value | |
308 | between the default below and the system's page size. The following | |
309 | command shows the current system's page size: `getconf PAGE_SIZE`. | |
310 | + | |
311 | Default values: | |
312 | + | |
c93eadad PP |
313 | * option:--userspace and option:--buffers-uid options: |
314 | {default_ust_uid_channel_subbuf_size} | |
315 | * option:--userspace and option:--buffers-pid options: | |
316 | {default_ust_pid_channel_subbuf_size} | |
317 | * option:--kernel option: {default_kernel_channel_subbuf_size} | |
318 | * `metadata` channel: {default_metadata_subbuf_size} | |
b4867b3b | 319 | |
b4867b3b PP |
320 | |
321 | Buffering scheme | |
322 | ~~~~~~~~~~~~~~~~ | |
323 | One of: | |
324 | ||
325 | option:--buffers-global:: | |
326 | Use shared sub-buffers for the whole system (only available with the | |
327 | option:--kernel option). | |
328 | ||
329 | option:--buffers-pid:: | |
330 | Use different sub-buffers for each traced process (only available | |
331 | with the the option:--userspace option). This is the default | |
332 | buffering scheme for user space channels. | |
333 | ||
334 | option:--buffers-uid:: | |
335 | Use shared sub-buffers for all the processes of the user running | |
336 | the command (only available with the option:--userspace option). | |
337 | ||
338 | ||
339 | Trace files | |
340 | ~~~~~~~~~~~ | |
341 | option:--tracefile-count='COUNT':: | |
342 | Limit the number of trace files created by this channel to | |
c93eadad PP |
343 | 'COUNT'. 0 means unlimited. Default: |
344 | {default_channel_tracefile_count}. | |
b4867b3b PP |
345 | + |
346 | Use this option in conjunction with the option:--tracefile-size option. | |
347 | + | |
348 | The file count within a stream is appended to each created trace | |
349 | file. If 'COUNT' files are created and more events need to be recorded, | |
350 | the first trace file of the stream is cleared and used again. | |
351 | ||
352 | option:--tracefile-size='SIZE':: | |
353 | Set the maximum size of each trace file written by | |
354 | this channel within a stream to 'SIZE' bytes. 0 means unlimited. | |
c93eadad | 355 | Default: {default_channel_tracefile_size}. |
b4867b3b PP |
356 | + |
357 | Note: traces generated with this option may inaccurately report | |
358 | discarded events as of CTF 1.8. | |
359 | ||
360 | ||
361 | Timers | |
362 | ~~~~~~ | |
961df0d0 PP |
363 | option:--monitor-timer:: |
364 | Set the channel's monitor timer's period to 'PERIODUS' µs. 0 means a | |
365 | disabled monitor timer. | |
366 | + | |
367 | Default values: | |
368 | + | |
369 | * option:--userspace and option:--buffers-uid options: | |
370 | {default_ust_uid_channel_monitor_timer} | |
371 | * option:--userspace and option:--buffers-pid options: | |
372 | {default_ust_pid_channel_monitor_timer} | |
373 | * option:--kernel option: {default_kernel_channel_monitor_timer} | |
374 | ||
b4867b3b PP |
375 | option:--read-timer:: |
376 | Set the channel's read timer's period to 'PERIODUS' µs. 0 means a | |
377 | disabled read timer. | |
378 | + | |
379 | Default values: | |
380 | + | |
c93eadad PP |
381 | * option:--userspace and option:--buffers-uid options: |
382 | {default_ust_uid_channel_read_timer} | |
383 | * option:--userspace and option:--buffers-pid options: | |
384 | {default_ust_pid_channel_read_timer} | |
385 | * option:--kernel option: {default_kernel_channel_read_timer} | |
386 | * `metadata` channel: {default_metadata_read_timer} | |
b4867b3b PP |
387 | |
388 | option:--switch-timer='PERIODUS':: | |
389 | Set the channel's switch timer's period to 'PERIODUS' µs. 0 means | |
c93eadad PP |
390 | a disabled switch timer. |
391 | + | |
392 | Default values: | |
393 | + | |
394 | * option:--userspace and option:--buffers-uid options: | |
395 | {default_ust_uid_channel_switch_timer} | |
396 | * option:--userspace and option:--buffers-pid options: | |
397 | {default_ust_pid_channel_switch_timer} | |
398 | * option:--kernel option: {default_kernel_channel_switch_timer} | |
399 | * `metadata` channel: {default_metadata_switch_timer} | |
b4867b3b | 400 | |
491d1539 MD |
401 | Timeouts |
402 | ~~~~~~~~ | |
db0f7697 PP |
403 | option:--blocking-timeout='TIMEOUTUS':: |
404 | Set the channel's blocking timeout value to 'TIMEOUTUS' µs | |
405 | for applications executed with a set `LTTNG_UST_ALLOW_BLOCKING` | |
406 | environment variable: | |
407 | + | |
408 | -- | |
409 | 0 (default):: | |
410 | Do not block. | |
411 | ||
412 | -1:: | |
413 | Block forever until room is available in the sub-buffer to write the | |
414 | event record. | |
415 | ||
416 | __n__, a positive value:: | |
417 | Wait for at most __n__ µs when trying to write into a sub-buffer. | |
418 | -- | |
419 | ||
b4867b3b PP |
420 | |
421 | include::common-cmd-help-options.txt[] | |
422 | ||
423 | ||
1076f2b7 PP |
424 | [[limitations]] |
425 | LIMITATIONS | |
426 | ----------- | |
427 | As of this version of LTTng, it is not possible to perform the following | |
428 | actions with the `lttng enable-channel` command: | |
429 | ||
430 | * Reconfigure a channel once it is created. | |
431 | * Re-enable a disabled channel once its tracing session has been active | |
432 | at least once. | |
433 | * Create a channel once its tracing session has been active | |
434 | at least once. | |
435 | * Create a user space channel with a given buffering scheme | |
436 | (option:--buffers-uid or option:--buffers-pid options) and create | |
437 | a second user space channel with a different buffering scheme in the | |
438 | same tracing session. | |
439 | ||
440 | ||
b4867b3b PP |
441 | include::common-cmd-footer.txt[] |
442 | ||
443 | ||
444 | SEE ALSO | |
445 | -------- | |
7c1a4458 | 446 | man:lttng-disable-channel(1), |
491d1539 MD |
447 | man:lttng(1), |
448 | man:lttng-ust(3) |