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] [--buffers-global]
      [--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] [--buffers-uid | --buffers-pid]
      [--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:

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 2.13.8, 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 buffering scheme (--buffers-uid or --buffers-pid options) and create a second user space channel with a different buffering scheme 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.

Buffering scheme

One of:

--buffers-global

Allocate a single set of ring buffers for the whole system.

Only available with the --kernel option.

As of LTTng 2.13.8, this is the default buffering scheme for the Linux kernel tracing domain, but this may change in the future.

--buffers-pid

Allocate one set of ring buffers (one per CPU) 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.

--buffers-uid

Allocate one set of ring buffers (one per CPU) 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 2.13.8, this is the default buffering scheme for the user space tracing domain, but this may change in the future.

Event record loss mode

--blocking-timeout=TIMEOUTUS

Set the channel’s blocking timeout value 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 2.13.8, 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 --buffers-uid options

4

--userspace and --buffers-pid options

4

--kernel and --buffers-global options

4

metadata channel

2

--output=TYPE

Set channel’s output type 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 --buffers-uid options

mmap

--userspace and --buffers-pid options

mmap

--kernel and --buffers-global 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 --buffers-uid options

524288

--userspace and --buffers-pid options

16384

--kernel and --buffers-global 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.
Note

Data 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 --buffers-uid options

1000000

--userspace and --buffers-pid options

1000000

--kernel and --buffers-global 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 --buffers-uid options

0

--userspace and --buffers-pid options

0

--kernel and --buffers-global 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 --buffers-uid options

0

--userspace and --buffers-pid options

0

--kernel and --buffers-global 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 buffering scheme in a specific recording session.

See the --session and --buffers-pid options.

$ lttng enable-channel --session=my-session --userspace \
                       --buffers-pid my-channel

Example 3. 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 4. 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 5. Enable two user space channels of a specific recording session.

$ lttng enable-channel --session=my-session --userspace \
                       canal-d,rds

Resources

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).

14 June 2021 LTTng 2.13.8 LTTng Manual