lttng-enable-channel man page

lttng-enable-channel — Create or enable LTTng channels


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] [--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]...


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: 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.

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 data.

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 option):

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 frequency.

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 and read timers

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 throughput.

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

By default, a 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.

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.


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

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:


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:


Set channel’s output type to TYPE.

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

Default values:

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 option).

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 option.

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

Default values:


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

Default values:

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.


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

Environment Variables


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


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.



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) command.


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




Command error


Undefined command


Fatal error


Command warning (something went wrong during the command)


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



This program is part of the LTTng-tools project.

LTTng-tools is distributed under the GNU General Public License version 2 <>. See the LICENSE <> file for details.


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.


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

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

Referenced By

lttng(1), lttng-add-context(1), lttng-create(1), lttng-disable-channel(1), lttng-metadata(1), lttng-regenerate(1).

02/21/2017 LTTng 2.9.4 LTTng Manual