lttng-enable-channel(1) — Linux manual page


LTTNG-ENABLE-CHANN(1)         LTTng Manual         LTTNG-ENABLE-CHANN(1)

NAME         top

       lttng-enable-channel - Create or enable LTTng channels

SYNOPSIS         top

       Create a Linux kernel channel:

       lttng [GENERAL OPTIONS] enable-channel --kernel
             [--overwrite] [--output=(mmap | splice)]
             [--subbuf-size=SIZE] [--num-subbuf=COUNT]
             [--switch-timer=PERIODUS] [--read-timer=PERIODUS]
             [--tracefile-size=SIZE] [--tracefile-count=COUNT]
             [--session=SESSION] CHANNEL

       Create a user space channel:

       lttng [GENERAL OPTIONS] enable-channel --userspace
             [--overwrite | --blocking-timeout=TIMEOUTUS] [--buffers-pid]
             [--subbuf-size=SIZE] [--num-subbuf=COUNT]
             [--switch-timer=PERIODUS] [--read-timer=PERIODUS]
             [--tracefile-size=SIZE] [--tracefile-count=COUNT]
             [--session=SESSION] CHANNEL

       Enable existing channel(s):

       lttng [GENERAL OPTIONS] enable-channel (--userspace | --kernel)
             [--session=SESSION] CHANNEL[,CHANNEL]...

DESCRIPTION         top

       The lttng enable-channel command can create a new channel, or
       enable one or more existing and disabled ones.

       A channel is the owner of sub-buffers holding recorded events.
       Event, rules, when created using lttng-enable-event(1), are
       always assigned to a channel. When creating a new channel, many
       parameters related to those sub-buffers can be fine-tuned. They
       are described in the subsections below.

       When CHANNEL does not name an existing channel, a channel named
       CHANNEL is created. Otherwise, the disabled channel named CHANNEL
       is enabled.

       Note that the lttng-enable-event(1) command can automatically
       create default channels when no channel exist.

       A channel is always contained in a tracing session (see
       lttng-create(1) for creating a tracing session). The session in
       which a channel is created using lttng enable-channel can be
       specified using the --session option. If the --session option is
       omitted, the current tracing session is targeted.

       Existing enabled channels can be disabled using
       lttng-disable-channel(1). Channels of a given session can be
       listed using lttng-list(1).

       See the LIMITATIONS section below for a list of limitations of
       this command to consider.

   Event loss modes
       LTTng tracers are non-blocking by default: when no empty
       sub-buffer exists, losing events is acceptable when the
       alternative would be to cause substantial delays in the
       instrumented application’s execution.

       LTTng privileges performance over integrity, aiming at perturbing
       the traced system as little as possible in order to make tracing
       of subtle race conditions and rare interrupt cascades possible.

       You can allow the user space tracer to block with a --blocking-
       timeout option set to a positive value or to inf, and with an
       application which is instrumented with LTTng-UST started with a
       set LTTNG_UST_ALLOW_BLOCKING environment variable. See
       lttng-ust(3) for more details.

       When it comes to losing events because no empty sub-buffer is
       available, the channel’s event loss mode, specified by one of the
       --discard and --overwrite options, determines what to do amongst:

           Drop the newest events until a sub-buffer is released.

           Clear the sub-buffer containing the oldest recorded events
           and start recording the newest events there. This mode is
           sometimes called flight recorder mode because it behaves like
           a flight recorder: always keep a fixed amount of the latest

       Which mechanism to choose depends on the context: prioritize the
       newest or the oldest events in the ring buffer?

       Beware that, in overwrite mode (--overwrite option), a whole
       sub-buffer is abandoned as soon as a new event doesn’t find an
       empty sub-buffer, whereas in discard mode (--discard option),
       only the event that doesn’t fit is discarded.

       Also note that a count of lost events is incremented and saved in
       the trace itself when an event is lost in discard mode, whereas
       no information is kept when a sub-buffer gets overwritten before
       being committed.

       The probability of losing events, if it is experience in a given
       context, can be reduced by fine-tuning the sub-buffers count and
       size (see next subsection).

   Sub-buffers count and size
       The --num-subbuf and --subbuf-size options respectively set the
       number of sub-buffers and their individual size when creating a
       new channel.

       Note that there is a noticeable tracer’s CPU overhead introduced
       when switching sub-buffers (marking a full one as consumable and
       switching to an empty one for the following events to be
       recorded). Knowing this, the following list presents a few
       practical situations along with how to configure sub-buffers for
       them when creating a channel in overwrite mode (--overwrite

       High event throughput
           In general, prefer bigger sub-buffers to lower the risk of
           losing events. Having bigger sub-buffers also ensures a lower
           sub-buffer switching frequency. The number of sub-buffers is
           only meaningful if the channel is enabled in overwrite mode:
           in this case, if a sub-buffer overwrite happens, the other
           sub-buffers are left unaltered.

       Low event throughput
           In general, prefer smaller sub-buffers since the risk of
           losing events is already low. Since events happen less
           frequently, the sub-buffer switching frequency should remain
           low and thus the tracer’s overhead should not be a problem.

       Low memory system
           If the target system has a low memory limit, prefer fewer
           first, then smaller sub-buffers. Even if the system is
           limited in memory, it is recommended to keep the sub-buffers
           as big as possible to avoid a high sub-buffer switching

       In discard mode (--discard option), the sub-buffers count
       parameter is pointless: using two sub-buffers and setting their
       size according to the requirements of the context is fine.

   Switch timer
       When a channel’s switch timer fires, a sub-buffer switch happens.
       This timer may be used to ensure that event data is consumed and
       committed to trace files periodically in case of a low event

       It’s also convenient when big sub-buffers are used to cope with
       sporadic high event throughput, even if the throughput is
       normally lower.

       Use the --switch-timer option to control the switch timer’s
       period of the channel to create.

   Read timer
       By default, an internal notification mechanism is used to signal
       a full sub-buffer so that it can be consumed. When such
       notifications must be avoided, for example in real-time
       applications, the channel’s read timer can be used instead. When
       the read timer fires, sub-buffers are checked for consumption
       when they are full.

       Use the --read-timer option to control the read timer’s period of
       the channel to create.

   Monitor timer
       When a channel’s monitor timer fires, its registered trigger
       conditions are evaluated using the current values of its
       properties (for example, the current usage of its sub-buffers).
       When a trigger condition is true, LTTng executes its associated
       action. The only type of action currently supported is to notify
       one or more user applications.

       See the installed C/C++ headers in lttng/action, lttng/condition,
       lttng/notification, and lttng/trigger to learn more about
       application notifications and triggers.

       Use the --monitor-timer option to control the monitor timer’s
       period of the channel to create.

   Buffering scheme
       In the user space tracing domain, two buffering schemes are
       available when creating a channel:

       Per-process buffering (--buffers-pid option)
           Keep one ring buffer per process.

       Per-user buffering (--buffers-uid option)
           Keep one ring buffer for all the processes of a single user.

       The per-process buffering scheme consumes more memory than the
       per-user option if more than one process is instrumented for
       LTTng-UST. However, per-process buffering ensures that one
       process having a high event throughput won’t fill all the shared
       sub-buffers, only its own.

       The Linux kernel tracing domain only has one available buffering
       scheme which is to use a single ring buffer for the whole system
       (--buffers-global option).

   Trace files limit and size
       By default, trace files can grow as large as needed. The maximum
       size of each trace file written by a channel can be set on
       creation using the --tracefile-size option. When such a trace
       file’s size reaches the channel’s fixed maximum size, another
       trace file is created to hold the next recorded events. A file
       count is appended to each trace file name in this case.

       If the --tracefile-size option is used, the maximum number of
       created trace files is unlimited. To limit them, the --tracefile-
       count option can be used. This option is always used in
       conjunction with the --tracefile-size option.

       For example, consider this command:

           $ lttng enable-channel --kernel --tracefile-size=4096 \
                                --tracefile-count=32 my-channel

       Here, for each stream, the maximum size of each trace file is 4
       kiB and there can be a maximum of 32 different files. When there
       is no space left in the last file, trace file rotation happens:
       the first file is cleared and new sub-buffers containing events
       are written there.

       LTTng does not guarantee that you can view the trace of an active
       tracing session (before you run the lttng-stop(1) command), even
       with multiple trace files, because LTTng could overwrite them at
       any moment, or some of them could be incomplete. You can archive
       a tracing session’s current trace chunk while the tracing session
       is active to obtain an unmanaged and self-contained LTTng trace:
       see lttng-rotate(1) and lttng-enable-rotation(1).

OPTIONS         top

       General options are described in lttng(1).

       One of:

       -k, --kernel
           Enable channel in the Linux kernel domain.

       -u, --userspace
           Enable channel in the user space domain.

       -s SESSION, --session=SESSION
           Create or enable channel in the tracing session named SESSION
           instead of the current tracing session.

   Event loss mode
           Set the channel’s blocking timeout value to TIMEOUTUS µs for
           instrumented applications executed with a set
           LTTNG_UST_ALLOW_BLOCKING environment variable:

           0 (default)
               Do not block (non-blocking mode).

               Block forever until room is available in the sub-buffer
               to write the event record.

           n, a positive value
               Wait for at most n µs when trying to write into a
               sub-buffer. After n µs, discard the event record.

           This option is only available with the --userspace option and
           without the --overwrite option.

       One of:

           Discard events when sub-buffers are full (default).

           Flight recorder mode: always keep a fixed amount of the
           latest data.

           Use COUNT sub-buffers. Rounded up to the next power of two.

           Default values:

           •   --userspace and --buffers-uid options: 4

           •   --userspace and --buffers-pid options: 4

           •   --kernel option: 4

           •   metadata channel: 2

           Set channel’s output type to TYPE.

           Available types: mmap (always available) and splice (only
           available with the --kernel option).

           Default values:

           •   --userspace and --buffers-uid options: mmap--userspace and --buffers-pid options: mmap--kernel option: splicemetadata channel: mmap

           Set the individual size of sub-buffers to SIZE bytes. The k
           (kiB), M (MiB), and G (GiB) suffixes are supported. Rounded
           up to the next power of two.

           The minimum sub-buffer size, for each tracer, is the maximum
           value between the default below and the system’s page size.
           The following command shows the current system’s page size:
           getconf PAGE_SIZE.

           Default values:

           •   --userspace and --buffers-uid options: 524288

           •   --userspace and --buffers-pid options: 16384

           •   --kernel option: 1048576

           •   metadata channel: 4096

   Buffering scheme
       One of:

           Use shared sub-buffers for the whole system (only available
           with the --kernel option).

           Use different sub-buffers for each traced process (only
           available with the the --userspace option). This is the
           default buffering scheme for user space channels.

           Use shared sub-buffers for all the processes of the user
           running the command (only available with the --userspace

   Trace files
           Limit the number of trace files created by this channel to
           COUNT. 0 means unlimited. Default: 0.

           Use this option in conjunction with the --tracefile-size

           The file count within a stream is appended to each created
           trace file. If COUNT files are created and more events need
           to be recorded, the first trace file of the stream is cleared
           and used again.

           Set the maximum size of each trace file written by this
           channel within a stream to SIZE bytes. 0 means unlimited.
           Default: 0.

           Note: traces generated with this option may inaccurately
           report discarded events as of CTF 1.8.

           Set the channel’s monitor timer’s period to PERIODUS µs. 0
           means a disabled monitor timer.

           Default values:

           •   --userspace and --buffers-uid options: 1000000

           •   --userspace and --buffers-pid options: 1000000

           •   --kernel option: 1000000

           Set the channel’s read timer’s period to PERIODUS µs. 0 means
           a disabled read timer.

           Default values:

           •   --userspace and --buffers-uid options: 0

           •   --userspace and --buffers-pid options: 0

           •   --kernel option: 200000

           •   metadata channel: 0

           Set the channel’s switch timer’s period to PERIODUS µs. 0
           means a disabled switch timer.

           Default values:

           •   --userspace and --buffers-uid options: 0

           •   --userspace and --buffers-pid options: 0

           •   --kernel option: 0

           •   metadata channel: 0

   Program information
       -h, --help
           Show command help.

           This option, like lttng-help(1), attempts to launch
           /usr/bin/man to view the command’s man page. The path to the
           man pager can be overridden by the LTTNG_MAN_BIN_PATH
           environment variable.

           List available command options.

LIMITATIONS         top

       As of this version of LTTng, it is not possible to perform the
       following actions with the lttng enable-channel command:

       •   Reconfigure a channel once it is created.

       •   Re-enable a disabled channel once its tracing session has
           been active at least once.

       •   Create a channel once its tracing session has been active at
           least once.

       •   Create a user space channel with a given buffering scheme
           (--buffers-uid or --buffers-pid options) and create a second
           user space channel with a different buffering scheme in the
           same tracing session.


           Set to 1 to abort the process after the first error is

           Overrides the $HOME environment variable. Useful when the
           user running the commands has a non-writable home directory.

           Absolute path to the man pager to use for viewing help
           information about LTTng commands (using lttng-help(1) or
           lttng COMMAND --help).

           Path in which the session.xsd session configuration XML
           schema may be found.

           Full session daemon binary path.

           The --sessiond-path option has precedence over this
           environment variable.

       Note that the lttng-create(1) command can spawn an LTTng session
       daemon automatically if none is running. See lttng-sessiond(8)
       for the environment variables influencing the execution of the
       session daemon.

FILES         top

           User LTTng runtime configuration.

           This is where the per-user current tracing session is stored
           between executions of lttng(1). The current tracing session
           can be set with lttng-set-session(1). See lttng-create(1) for
           more information about tracing sessions.

           Default output directory of LTTng traces. This can be
           overridden with the --output option of the lttng-create(1)

           User LTTng runtime and configuration directory.

           Default location of saved user tracing sessions (see
           lttng-save(1) and lttng-load(1)).

           System-wide location of saved tracing sessions (see
           lttng-save(1) and lttng-load(1)).


           $LTTNG_HOME defaults to $HOME when not explicitly set.

EXIT STATUS         top


           Command error

           Undefined command

           Fatal error

           Command warning (something went wrong during the command)

BUGS         top

       If you encounter any issue or usability problem, please report it
       on the LTTng bug tracker <

RESOURCES         top

       •   LTTng project website <>

       •   LTTng documentation <>

       •   Git repositories <>

       •   GitHub organization <>

       •   Continuous integration <>

       •   Mailing list <> for support and

       •   IRC channel <irc://>: #lttng on

COPYRIGHTS         top

       This program is part of the LTTng-tools project.

       LTTng-tools is distributed under the GNU General Public License
       version 2 <
       licenses/gpl-2.0.en.html>. See the LICENSE
       <> file
       for details.

THANKS         top

       Special thanks to Michel Dagenais and the DORSAL laboratory
       <> at École Polytechnique de
       Montréal for the LTTng journey.

       Also thanks to the Ericsson teams working on tracing which helped
       us greatly with detailed bug reports and unusual test cases.

AUTHORS         top

       LTTng-tools was originally written by Mathieu Desnoyers, Julien
       Desfossez, and David Goulet. More people have since contributed
       to it.

       LTTng-tools is currently maintained by Jérémie Galarneau

SEE ALSO         top

       lttng-disable-channel(1), lttng(1), lttng-ust(3)

COLOPHON         top

       This page is part of the LTTng-Tools (    LTTng tools) project.
       Information about the project can be found at 
       ⟨⟩.  It is not known how to report bugs for this
       man page; if you know, please send a mail to
       This page was obtained from the project's upstream Git repository
       ⟨git://⟩ on 2019-11-19.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2019-11-14.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

LTTng 2.12.0-pre               10/29/2018          LTTNG-ENABLE-CHANN(1)

Pages that refer to this page: lttng(1)lttng-add-context(1)lttng-create(1)lttng-disable-channel(1)lttng-disable-rotation(1)lttng-enable-rotation(1)lttng-metadata(1)lttng-regenerate(1)lttng-rotate(1)lttng-ust(3)