lttng-enable-channel — Create or enable LTTng channels
Create a Linux kernel channel:
lttng [GENERAL OPTIONS] enable-channel
Create a user space channel:
lttng [GENERAL OPTIONS] enable-channel
Enable existing channel(s):
lttng [GENERAL OPTIONS] enable-channel (
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
option is omitted, the current tracing session is targeted.
See the LIMITATIONS section below for a list of limitations of this command to consider.
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.
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).
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
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.
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.
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.
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.
In the user space tracing domain, two buffering schemes are available when creating a channel:
Keep one ring buffer per process.
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
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.
--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
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.
Use COUNT sub-buffers. Rounded up to the next power of two.
Set the individual size of sub-buffers to SIZE bytes.
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:
Set channel’s output type to TYPE.
mmap (always available) and
splice (only available
Use shared sub-buffers for the whole system (only available with the
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
Limit the number of trace files created by this channel to COUNT. 0 means unlimited. Default: 0.
Use this option in conjunction with the
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.
Set the channel’s switch timer’s period to PERIODUS µs. 0 means a disabled switch timer.
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-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 encountered.
$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.
--sessiond-path option has precedence over this
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
--output option of the lttng-create(1)
User LTTng runtime and configuration directory.
$LTTNG_HOME defaults to
$HOME when not explicitly set.
Command warning (something went wrong during the command)
If you encounter any issue or usability problem, please report it on the LTTng bug tracker.
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.