lttng-enable-channel(1)

1LTTNG-ENABLE-CHANN(1)            LTTng Manual            LTTNG-ENABLE-CHANN(1)
2
3
4

NAME

6       lttng-enable-channel - Create or enable LTTng channels
7

SYNOPSIS

9       Create a Linux kernel channel:
10
11       lttng [GENERAL OPTIONS] enable-channel --kernel
12             [--overwrite] [--output=(mmap | splice)]
13             [--subbuf-size=SIZE] [--num-subbuf=COUNT]
14             [--switch-timer=PERIODUS] [--read-timer=PERIODUS]
15             [--monitor-timer=PERIODUS]
16             [--tracefile-size=SIZE] [--tracefile-count=COUNT]
17             [--session=SESSION] CHANNEL
18
19       Create a user space channel:
20
21       lttng [GENERAL OPTIONS] enable-channel --userspace
22             [--overwrite | --blocking-timeout=TIMEOUTUS] [--buffers-pid]
23             [--subbuf-size=SIZE] [--num-subbuf=COUNT]
24             [--switch-timer=PERIODUS] [--read-timer=PERIODUS]
25             [--monitor-timer=PERIODUS]
26             [--tracefile-size=SIZE] [--tracefile-count=COUNT]
27             [--session=SESSION] CHANNEL
28
29       Enable existing channel(s):
30
31       lttng [GENERAL OPTIONS] enable-channel (--userspace | --kernel)
32             [--session=SESSION] CHANNEL[,CHANNEL]...
33

DESCRIPTION

35       The lttng enable-channel command can create a new channel, or enable
36       one or more existing and disabled ones.
37
38       A channel is the owner of sub-buffers holding recorded events. Event,
39       rules, when created using lttng-enable-event(1), are always assigned to
40       a channel. When creating a new channel, many parameters related to
41       those sub-buffers can be fine-tuned. They are described in the
42       subsections below.
43
44       When CHANNEL does not name an existing channel, a channel named CHANNEL
45       is created. Otherwise, the disabled channel named CHANNEL is enabled.
46
47       Note that the lttng-enable-event(1) command can automatically create
48       default channels when no channel exist.
49
50       A channel is always contained in a tracing session (see lttng-create(1)
51       for creating a tracing session). The session in which a channel is
52       created using lttng enable-channel can be specified using the --session
53       option. If the --session option is omitted, the current tracing session
54       is targeted.
55
56       Existing enabled channels can be disabled using lttng-disable-
57       channel(1). Channels of a given session can be listed using lttng-
58       list(1).
59
60       See the LIMITATIONS section below for a list of limitations of this
61       command to consider.
62
63   Event loss modes
64       LTTng tracers are non-blocking by default: when no empty sub-buffer
65       exists, losing events is acceptable when the alternative would be to
66       cause substantial delays in the instrumented application’s execution.
67
68       LTTng privileges performance over integrity, aiming at perturbing the
69       traced system as little as possible in order to make tracing of subtle
70       race conditions and rare interrupt cascades possible.
71
72       You can allow the user space tracer to block with a --blocking-timeout
73       option set to a positive value or to inf, and with an application which
74       is instrumented with LTTng-UST started with a set
75       LTTNG_UST_ALLOW_BLOCKING environment variable. See lttng-ust(3) for
76       more details.
77
78       When it comes to losing events because no empty sub-buffer is
79       available, the channel’s event loss mode, specified by one of the
80       --discard and --overwrite options, determines what to do amongst:
81
82       Discard
83           Drop the newest events until a sub-buffer is released.
84
85       Overwrite
86           Clear the sub-buffer containing the oldest recorded events and
87           start recording the newest events there. This mode is sometimes
88           called flight recorder mode because it behaves like a flight
89           recorder: always keep a fixed amount of the latest data.
90
91       Which mechanism to choose depends on the context: prioritize the newest
92       or the oldest events in the ring buffer?
93
94       Beware that, in overwrite mode (--overwrite option), a whole sub-buffer
95       is abandoned as soon as a new event doesn’t find an empty sub-buffer,
96       whereas in discard mode (--discard option), only the event that doesn’t
97       fit is discarded.
98
99       Also note that a count of lost events is incremented and saved in the
100       trace itself when an event is lost in discard mode, whereas no
101       information is kept when a sub-buffer gets overwritten before being
102       committed.
103
104       The probability of losing events, if it is experience in a given
105       context, can be reduced by fine-tuning the sub-buffers count and size
106       (see next subsection).
107
108   Sub-buffers count and size
109       The --num-subbuf and --subbuf-size options respectively set the number
110       of sub-buffers and their individual size when creating a new channel.
111
112       Note that there is a noticeable tracer’s CPU overhead introduced when
113       switching sub-buffers (marking a full one as consumable and switching
114       to an empty one for the following events to be recorded). Knowing this,
115       the following list presents a few practical situations along with how
116       to configure sub-buffers for them when creating a channel in overwrite
117       mode (--overwrite option):
118
119       High event throughput
120           In general, prefer bigger sub-buffers to lower the risk of losing
121           events. Having bigger sub-buffers also ensures a lower sub-buffer
122           switching frequency. The number of sub-buffers is only meaningful
123           if the channel is enabled in overwrite mode: in this case, if a
124           sub-buffer overwrite happens, the other sub-buffers are left
125           unaltered.
126
127       Low event throughput
128           In general, prefer smaller sub-buffers since the risk of losing
129           events is already low. Since events happen less frequently, the
130           sub-buffer switching frequency should remain low and thus the
131           tracer’s overhead should not be a problem.
132
133       Low memory system
134           If the target system has a low memory limit, prefer fewer first,
135           then smaller sub-buffers. Even if the system is limited in memory,
136           it is recommended to keep the sub-buffers as big as possible to
137           avoid a high sub-buffer switching frequency.
138
139       In discard mode (--discard option), the sub-buffers count parameter is
140       pointless: using two sub-buffers and setting their size according to
141       the requirements of the context is fine.
142
143   Switch timer
144       When a channel’s switch timer fires, a sub-buffer switch happens. This
145       timer may be used to ensure that event data is consumed and committed
146       to trace files periodically in case of a low event throughput.
147
148       It’s also convenient when big sub-buffers are used to cope with
149       sporadic high event throughput, even if the throughput is normally
150       lower.
151
152       Use the --switch-timer option to control the switch timer’s period of
153       the channel to create.
154
155   Read timer
156       By default, an internal notification mechanism is used to signal a full
157       sub-buffer so that it can be consumed. When such notifications must be
158       avoided, for example in real-time applications, the channel’s read
159       timer can be used instead. When the read timer fires, sub-buffers are
160       checked for consumption when they are full.
161
162       Use the --read-timer option to control the read timer’s period of the
163       channel to create.
164
165   Monitor timer
166       When a channel’s monitor timer fires, its registered trigger conditions
167       are evaluated using the current values of its properties (for example,
168       the current usage of its sub-buffers). When a trigger condition is
169       true, LTTng executes its associated action. The only type of action
170       currently supported is to notify one or more user applications.
171
172       See the installed C/C++ headers in lttng/action, lttng/condition,
173       lttng/notification, and lttng/trigger to learn more about application
174       notifications and triggers.
175
176       Use the --monitor-timer option to control the monitor timer’s period of
177       the channel to create.
178
179   Buffering scheme
180       In the user space tracing domain, two buffering schemes are available
181       when creating a channel:
182
183       Per-process buffering (--buffers-pid option)
184           Keep one ring buffer per process.
185
186       Per-user buffering (--buffers-uid option)
187           Keep one ring buffer for all the processes of a single user.
188
189       The per-process buffering scheme consumes more memory than the per-user
190       option if more than one process is instrumented for LTTng-UST. However,
191       per-process buffering ensures that one process having a high event
192       throughput won’t fill all the shared sub-buffers, only its own.
193
194       The Linux kernel tracing domain only has one available buffering scheme
195       which is to use a single ring buffer for the whole system (--buffers-
196       global option).
197
198   Trace files limit and size
199       By default, trace files can grow as large as needed. The maximum size
200       of each trace file written by a channel can be set on creation using
201       the --tracefile-size option. When such a trace file’s size reaches the
202       channel’s fixed maximum size, another trace file is created to hold the
203       next recorded events. A file count is appended to each trace file name
204       in this case.
205
206       If the --tracefile-size option is used, the maximum number of created
207       trace files is unlimited. To limit them, the --tracefile-count option
208       can be used. This option is always used in conjunction with the
209       --tracefile-size option.
210
211       For example, consider this command:
212
213           $ lttng enable-channel --kernel --tracefile-size=4096 \
214                                --tracefile-count=32 my-channel
215
216       Here, for each stream, the maximum size of each trace file is 4 kiB and
217       there can be a maximum of 32 different files. When there is no space
218       left in the last file, trace file rotation happens: the first file is
219       cleared and new sub-buffers containing events are written there.
220

OPTIONS

222       General options are described in lttng(1).
223
224   Domain
225       One of:
226
227       -k, --kernel
228           Enable channel in the Linux kernel domain.
229
230       -u, --userspace
231           Enable channel in the user space domain.
232
233   Target
234       -s SESSION, --session=SESSION
235           Create or enable channel in the tracing session named SESSION
236           instead of the current tracing session.
237
238   Event loss mode
239       --blocking-timeout=TIMEOUTUS
240           Set the channel’s blocking timeout value to TIMEOUTUS µs for
241           instrumented applications executed with a set
242           LTTNG_UST_ALLOW_BLOCKING environment variable:
243
244           0 (default)
245               Do not block (non-blocking mode).
246
247           inf
248               Block forever until room is available in the sub-buffer to
249               write the event record.
250
251           n, a positive value
252               Wait for at most n µs when trying to write into a sub-buffer.
253               After n µs, discard the event record.
254
255           This option is only available with the --userspace option and
256           without the --overwrite option.
257
258       One of:
259
260       --discard
261           Discard events when sub-buffers are full (default).
262
263       --overwrite
264           Flight recorder mode: always keep a fixed amount of the latest
265           data.
266
267   Sub-buffers
268       --num-subbuf=COUNT
269           Use COUNT sub-buffers. Rounded up to the next power of two.
270
271           Default values:
272
273           ·   --userspace and --buffers-uid options: 4
274
275           ·   --userspace and --buffers-pid options: 4
276
277           ·   --kernel option: 4
278
279           ·   metadata channel: 2
280
281       --output=TYPE
282           Set channel’s output type to TYPE.
283
284           Available types: mmap (always available) and splice (only available
285           with the --kernel option).
286
287           Default values:
288
289           ·   --userspace and --buffers-uid options: mmap
290
291           ·   --userspace and --buffers-pid options: mmap
292
293           ·   --kernel option: splice
294
295           ·   metadata channel: mmap
296
297       --subbuf-size=SIZE
298           Set the individual size of sub-buffers to SIZE bytes. The k (kiB),
299           M (MiB), and G (GiB) suffixes are supported. Rounded up to the next
300           power of two.
301
302           The minimum sub-buffer size, for each tracer, is the maximum value
303           between the default below and the system’s page size. The following
304           command shows the current system’s page size: getconf PAGE_SIZE.
305
306           Default values:
307
308           ·   --userspace and --buffers-uid options: 524288
309
310           ·   --userspace and --buffers-pid options: 16384
311
312           ·   --kernel option: 1048576
313
314           ·   metadata channel: 4096
315
316   Buffering scheme
317       One of:
318
319       --buffers-global
320           Use shared sub-buffers for the whole system (only available with
321           the --kernel option).
322
323       --buffers-pid
324           Use different sub-buffers for each traced process (only available
325           with the the --userspace option). This is the default buffering
326           scheme for user space channels.
327
328       --buffers-uid
329           Use shared sub-buffers for all the processes of the user running
330           the command (only available with the --userspace option).
331
332   Trace files
333       --tracefile-count=COUNT
334           Limit the number of trace files created by this channel to COUNT. 0
335           means unlimited. Default: 0.
336
337           Use this option in conjunction with the --tracefile-size option.
338
339           The file count within a stream is appended to each created trace
340           file. If COUNT files are created and more events need to be
341           recorded, the first trace file of the stream is cleared and used
342           again.
343
344       --tracefile-size=SIZE
345           Set the maximum size of each trace file written by this channel
346           within a stream to SIZE bytes. 0 means unlimited. Default: 0.
347
348           Note: traces generated with this option may inaccurately report
349           discarded events as of CTF 1.8.
350
351   Timers
352       --monitor-timer
353           Set the channel’s monitor timer’s period to PERIODUS µs. 0 means a
354           disabled monitor timer.
355
356           Default values:
357
358           ·   --userspace and --buffers-uid options: 1000000
359
360           ·   --userspace and --buffers-pid options: 1000000
361
362           ·   --kernel option: 1000000
363
364       --read-timer
365           Set the channel’s read timer’s period to PERIODUS µs. 0 means a
366           disabled read timer.
367
368           Default values:
369
370           ·   --userspace and --buffers-uid options: 0
371
372           ·   --userspace and --buffers-pid options: 0
373
374           ·   --kernel option: 200000
375
376           ·   metadata channel: 0
377
378       --switch-timer=PERIODUS
379           Set the channel’s switch timer’s period to PERIODUS µs. 0 means a
380           disabled switch timer.
381
382           Default values:
383
384           ·   --userspace and --buffers-uid options: 0
385
386           ·   --userspace and --buffers-pid options: 0
387
388           ·   --kernel option: 0
389
390           ·   metadata channel: 0
391
392   Program information
393       -h, --help
394           Show command help.
395
396           This option, like lttng-help(1), attempts to launch /usr/bin/man to
397           view the command’s man page. The path to the man pager can be
398           overridden by the LTTNG_MAN_BIN_PATH environment variable.
399
400       --list-options
401           List available command options.
402

LIMITATIONS

404       As of this version of LTTng, it is not possible to perform the
405       following actions with the lttng enable-channel command:
406
407       ·   Reconfigure a channel once it is created.
408
409       ·   Re-enable a disabled channel once its tracing session has been
410           active at least once.
411
412       ·   Create a channel once its tracing session has been active at least
413           once.
414
415       ·   Create a user space channel with a given buffering scheme
416           (--buffers-uid or --buffers-pid options) and create a second user
417           space channel with a different buffering scheme in the same tracing
418           session.
419

ENVIRONMENT VARIABLES

421       LTTNG_ABORT_ON_ERROR
422           Set to 1 to abort the process after the first error is encountered.
423
424       LTTNG_HOME
425           Overrides the $HOME environment variable. Useful when the user
426           running the commands has a non-writable home directory.
427
428       LTTNG_MAN_BIN_PATH
429           Absolute path to the man pager to use for viewing help information
430           about LTTng commands (using lttng-help(1) or lttng COMMAND --help).
431
432       LTTNG_SESSION_CONFIG_XSD_PATH
433           Path in which the session.xsd session configuration XML schema may
434           be found.
435
436       LTTNG_SESSIOND_PATH
437           Full session daemon binary path.
438
439           The --sessiond-path option has precedence over this environment
440           variable.
441
442       Note that the lttng-create(1) command can spawn an LTTng session daemon
443       automatically if none is running. See lttng-sessiond(8) for the
444       environment variables influencing the execution of the session daemon.
445

FILES

447       $LTTNG_HOME/.lttngrc
448           User LTTng runtime configuration.
449
450           This is where the per-user current tracing session is stored
451           between executions of lttng(1). The current tracing session can be
452           set with lttng-set-session(1). See lttng-create(1) for more
453           information about tracing sessions.
454
455       $LTTNG_HOME/lttng-traces
456           Default output directory of LTTng traces. This can be overridden
457           with the --output option of the lttng-create(1) command.
458
459       $LTTNG_HOME/.lttng
460           User LTTng runtime and configuration directory.
461
462       $LTTNG_HOME/.lttng/sessions
463           Default location of saved user tracing sessions (see lttng-save(1)
464           and lttng-load(1)).
465
466       /usr/local/etc/lttng/sessions
467           System-wide location of saved tracing sessions (see lttng-save(1)
468           and lttng-load(1)).
469
470           Note
471           $LTTNG_HOME defaults to $HOME when not explicitly set.
472

EXIT STATUS

474       0
475           Success
476
477       1
478           Command error
479
480       2
481           Undefined command
482
483       3
484           Fatal error
485
486       4
487           Command warning (something went wrong during the command)
488

BUGS

490       If you encounter any issue or usability problem, please report it on
491       the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-tools>.
492

RESOURCES

494       ·   LTTng project website <http://lttng.org>
495
496       ·   LTTng documentation <http://lttng.org/docs>
497
498       ·   Git repositories <http://git.lttng.org>
499
500       ·   GitHub organization <http://github.com/lttng>
501
502       ·   Continuous integration <http://ci.lttng.org/>
503
504       ·   Mailing list <http://lists.lttng.org> for support and development:
505           lttng-dev@lists.lttng.org
506
507       ·   IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net
508

COPYRIGHTS

510       This program is part of the LTTng-tools project.
511
512       LTTng-tools is distributed under the GNU General Public License version
513       2 <http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html>. See the
514       LICENSE <https://github.com/lttng/lttng-tools/blob/master/LICENSE> file
515       for details.
516

THANKS

518       Special thanks to Michel Dagenais and the DORSAL laboratory
519       <http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal for
520       the LTTng journey.
521
522       Also thanks to the Ericsson teams working on tracing which helped us
523       greatly with detailed bug reports and unusual test cases.
524

AUTHORS

526       LTTng-tools was originally written by Mathieu Desnoyers, Julien
527       Desfossez, and David Goulet. More people have since contributed to it.
528
529       LTTng-tools is currently maintained by Jérémie Galarneau
530       <mailto:jeremie.galarneau@efficios.com>.
531