lttng-enable-channel - Man Page
Create or enable LTTng channels
Synopsis
Create a Linux kernel channel:
lttng [GENERAL OPTIONS] enable-channel --kernel [--discard | --overwrite] [--output=(mmap | splice)] [--subbuf-size=SIZE] [--num-subbuf=COUNT] [--switch-timer=PERIODUS] [--read-timer=PERIODUS] [--monitor-timer=PERIODUS] [--buffer-ownership=system] [--buffer-allocation=per-cpu] [--tracefile-size=SIZE [--tracefile-count=COUNT]] [--session=SESSION] CHANNEL
Create a user space channel:
lttng [GENERAL OPTIONS] enable-channel --userspace [--overwrite | [--discard] --blocking-timeout=TIMEOUTUS] [--output=mmap] [--buffer-ownership=(user | process)] [--buffer-allocation=(per-cpu | per-channel)] [--subbuf-size=SIZE] [--num-subbuf=COUNT] [--switch-timer=PERIODUS] [--read-timer=PERIODUS] [--monitor-timer=PERIODUS] [--tracefile-size=SIZE [--tracefile-count=COUNT]] [--session=SESSION] CHANNEL
Enable channel(s):
lttng [GENERAL OPTIONS] enable-channel (--userspace | --kernel) [--session=SESSION] CHANNEL[,CHANNEL]...
Description
The lttng enable-channel command does one of:
- Create a channel named CHANNEL.
- Enable one or more disabled channels named CHANNEL (non-option argument, comma-separated).
See lttng-concepts(7) to learn more about channels.
The channel(s) to create or enable belong to:
- With the --session=SESSION option
The recording session named SESSION.
- Without the --session option
The current recording session (see lttng-concepts(7) to learn more about the current recording session).
- Note
The lttng-enable-event(1) command can automatically create a default channel when no channel exists for the provided tracing domain.
See the “Examples” section below for usage examples.
List the channels of a given recording session with the lttng-list(1) and lttng-status(1) commands.
Disable an enabled channel with the lttng-disable-channel(1) command.
Important
As of LTTng 2.14.0, you may NOT perform the following operations with the enable-channel command:
- Change an attribute of an existing channel.
- Enable a disabled channel once its recording session has been active (started; see lttng-start(1)) at least once.
- Create a channel once its recording session has been active at least once.
- Create a user space channel with a given buffer ownership model (--buffer-ownership set to user or process) and create a second user space channel with a different buffer ownership model in the same recording session.
Options
See lttng(1) for GENERAL OPTIONS.
Tracing domain
One of:
- -k, --kernel
Create or enable channels in the Linux kernel domain.
- -u, --userspace
Create or enable channels in the user space domain.
Recording target
- -s SESSION, --session=SESSION
Create or enable channels in the recording session named SESSION instead of the current recording session.
Buffer ownership model
One of:
- --buffers-global
Equivalent to --buffer-ownership=system.
Deprecated.
- --buffer-ownership=MODEL
Depending on MODEL:
- process
Allocate one set of ring buffers (one per CPU or one for the whole channel, depending on --buffer-allocation) for each instrumented process of:
If you connect to the root session daemon
All Unix users.
See the “Session daemon connection” section of lttng(1) to learn how a user application connects to a session daemon.
- Otherwise
Your Unix user.
Only available with the --userspace option.
- system
Allocate a single set of ring buffers (one per CPU) for the whole system.
Only available with the --kernel option.
As of LTTng 2.14.0, this is the default buffer ownership model for the Linux kernel tracing domain, but this may change in the future.
- user
Allocate one set of ring buffers (one per CPU or one for the whole channel, depending on --buffer-allocation) shared by all the instrumented processes of:
If you connect to the root session daemon
Each Unix user.
See the “Session daemon connection” section of lttng(1) to learn how a user application connects to a session daemon.
- Otherwise
Your Unix user.
Only available with the --userspace option.
As of LTTng 2.14.0, this is the default buffer ownership model for the user space tracing domain, but this may change in the future.
- --buffers-pid
Equivalent to --buffer-ownership=process.
Deprecated.
- --buffers-uid
Equivalent to --buffer-ownership=user.
Deprecated.
Buffer allocation policy
- --buffer-allocation=POLICY
Depending on POLICY, allocate one ring buffer for:
- per-channel
The whole channel.
Setting --buffer-ownership to user or process determines what entity gets a channel-wise ring buffer.
Only available with the --userspace option.
- per-cpu
Each CPU.
Setting --buffer-ownership to user, process, or system determines what entity gets a set of per-CPU ring buffers.
Using this buffer allocation policy implies adding the cpu_id context field, without any way to remove it, for the created channel, similar to running:
$ lttng add-context --session=SESSION --channel=CHANNEL \ --type=cpu_id
As of LTTng 2.14.0, this is the default buffer allocation policy, but this may change in the future.
Event record loss mode
- --blocking-timeout=TIMEOUTUS
Set the blocking timeout value of the channel to TIMEOUTUS µs for instrumented applications executed with a set LTTNG_UST_ALLOW_BLOCKING environment variable.
TIMEOUTUS is one of:
- 0 (default)
Do not block (non-blocking mode).
- inf
Block forever until a sub-buffer is available to write the event record.
- N, a positive value
Wait for at most N µs when trying to write to a sub-buffer. After N µs, discard the event record.
This option is only available with both the --userspace and --discard options.
One of:
- --discard
Discard event records when there’s no available sub-buffer.
As of LTTng 2.14.0, this is the default event record loss mode, but this may change in the future.
- --overwrite
Overwrite the whole sub-buffer containing the oldest event records when there’s no available sub-buffer (flight recorder mode).
Sub-buffers
- --num-subbuf=COUNT
Use COUNT sub-buffers per ring buffer.
The effective value is COUNT rounded up to the next power of two.
Default values:
- --userspace and --buffer-ownership=user options
4
- --userspace and --buffer-ownership=process options
4
- --kernel and --buffer-ownership=system options
4
metadata channel
2
- --output=TYPE
Set the output type of the channel to TYPE.
TYPE is one of:
- mmap
Share ring buffers between the tracer and the consumer daemon with the mmap(2) system call.
- splice
Share ring buffers between the tracer and the consumer daemon with the splice(2) system call.
Only available with the --kernel option.
Default values:
- --userspace and --buffer-ownership=user options
mmap
- --userspace and --buffer-ownership=process options
mmap
- --kernel and --buffer-ownership=system options
splice
- metadata channel
mmap
- --subbuf-size=SIZE
Set the size of each sub-buffer to SIZE bytes.
The effective value is SIZE rounded up to the next power of two.
The k (KiB), M (MiB), and G (GiB) suffixes are supported.
The minimum sub-buffer size, for each tracer, is the maximum value between the default below and the system page size (see getconf(1) with the PAGE_SIZE variable).
Default values:
- --userspace and --buffer-ownership=user options
524288
- --userspace and --buffer-ownership=process options
16384
- --kernel and --buffer-ownership=system options
1048576
metadata channel
4096
Trace files
- --tracefile-count=COUNT
Limit the number of trace files which LTTng writes for this channel to COUNT.
COUNT set to 0 means “unlimited”.
Default: 0.
You must also use the --tracefile-size option with this option.
- --tracefile-size=SIZE
Set the maximum size of each trace file which LTTng writes for this channel to SIZE bytes.
SIZE set to 0 means “unlimited”.
Default: 0.
NoteData streams which LTTng writes for a channel configured with this option may inaccurately report discarded event records as of CTF 1.8.
Timers
- --monitor-timer=PERIODUS
Set the period of the monitor timer of the channel to PERIODUS µs.
Set PERIODUS to 0 to disable the monitor timer.
Default values:
- --userspace and --buffer-ownership=user options
1000000
- --userspace and --buffer-ownership=process options
1000000
- --kernel and --buffer-ownership=system options
1000000
- --read-timer=PERIODUS
Set the period of the read timer of the channel to PERIODUS µs.
Set PERIODUS to 0 to disable the read timer.
Default values:
- --userspace and --buffer-ownership=user options
0
- --userspace and --buffer-ownership=process options
0
- --kernel and --buffer-ownership=system options
200000
metadata channel
0
- --switch-timer=PERIODUS
Set the period of the switch timer of the channel to PERIODUS µs.
Set PERIODUS to 0 to disable the switch timer.
Default values:
- --userspace and --buffer-ownership=user options
0
- --userspace and --buffer-ownership=process options
0
- --kernel and --buffer-ownership=system options
0
metadata channel
0
Program information
- -h, --help
Show help.
This option attempts to launch /usr/bin/man to view this manual page. Override the manual pager path with the LTTNG_MAN_BIN_PATH environment variable.
- --list-options
List available command options and quit.
Exit Status
- 0
Success
- 1
Command error
- 2
Undefined command
- 3
Fatal error
- 4
Command warning (something went wrong during the command)
Environment
- LTTNG_ABORT_ON_ERROR
Set to 1 to abort the process after the first error is encountered.
- LTTNG_HOME
Path to the LTTng home directory.
Defaults to $HOME.
Useful when the Unix user running the commands has a non-writable home directory.
- LTTNG_MAN_BIN_PATH
Absolute path to the manual pager to use to read the LTTng command-line help (with lttng-help(1) or with the --help option) instead of /usr/bin/man.
- LTTNG_SESSION_CONFIG_XSD_PATH
Path to the directory containing the session.xsd recording session configuration XML schema.
- LTTNG_SESSIOND_PATH
Absolute path to the LTTng session daemon binary (see lttng-sessiond(8)) to spawn from the lttng-create(1) command.
The --sessiond-path general option overrides this environment variable.
Files
- $LTTNG_HOME/.lttngrc
Unix user’s LTTng runtime configuration.
This is where LTTng stores the name of the Unix user’s current recording session between executions of lttng(1). lttng-create(1) and lttng-set-session(1) set the current recording session.
- $LTTNG_HOME/lttng-traces
Default output directory of LTTng traces in local and snapshot modes.
Override this path with the --output option of the lttng-create(1) command.
- $LTTNG_HOME/.lttng
Unix user’s LTTng runtime and configuration directory.
- $LTTNG_HOME/.lttng/sessions
Default directory containing the Unix user’s saved recording session configurations (see lttng-save(1) and lttng-load(1)).
- /usr/local/etc/lttng/sessions
Directory containing the system-wide saved recording session configurations (see lttng-save(1) and lttng-load(1)).
Note
$LTTNG_HOME defaults to the value of the HOME environment variable.
Examples
Example 1. Create a Linux kernel channel with default attributes in the current recording session.
The following command line only creates a new channel if my-channel doesn’t name an existing Linux kernel channel in the current recording session.
$ lttng enable-channel --kernel my-channel
Example 2. Create a user space channel with a per-process buffer ownership model in a specific recording session.
See the --session and --buffer-ownership options.
$ lttng enable-channel --session=my-session --userspace \ --buffer-ownership=process my-channel
Example 3. Create a user space channel with a per-user buffer ownership model (default) and a per-channel buffer allocation policy.
See the --buffer-allocation option.
$ lttng enable-channel --userspace \ --buffer-allocation=per-channel my-channel
Example 4. Create a Linux kernel channel in the current recording session with four 32-MiB sub-buffers per ring buffer.
See the --num-subbuf and --subbuf-size options.
$ lttng enable-channel --kernel my-channel \ --num-subbuf=4 --subbuf-size=32M
Example 5. Create a user space channel in the current recording session with trace file rotation.
See the --tracefile-count and --tracefile-size options.
$ lttng enable-channel --userspace my-channel \ --tracefile-count=16 --tracefile-size=8M
Example 6. Enable two user space channels of a specific recording session.
$ lttng enable-channel --session=my-session --userspace \ canal-d,rds
Resources
- LTTng project website <https://lttng.org>
- LTTng documentation <https://lttng.org/docs>
- LTTng bug tracker <https://bugs.lttng.org>
- Git repositories <https://git.lttng.org>
- GitHub organization <https://github.com/lttng>
- Continuous integration <https://ci.lttng.org/>
- Mailing list <https://lists.lttng.org/> for support and development: lttng-dev@lists.lttng.org
- IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net
- Mastodon <https://mastodon.social/@lttng>
Copyright
This program is part of the LTTng-tools project.
LTTng-tools is distributed under the GNU General Public License version 2 <http://www.gnu.org/licenses/old-licenses/gpl-2.0.en.html>. See the LICENSE <https://github.com/lttng/lttng-tools/blob/master/LICENSE> file for details.
Thanks
Special thanks to Michel Dagenais and the DORSAL laboratory <http://www.dorsal.polymtl.ca/> 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.
See Also
lttng(1), lttng-disable-channel(1), lttng-list(1), lttng-concepts(7)
Referenced By
lttng(1), lttng-add-context(1), lttng-concepts(7), lttng-create(1), lttng-disable-channel(1), lttng-enable-event(1), lttng-enable-rotation(1), lttng-regenerate(1), lttng-ust(3).