lttng-concepts(7)
=================
:revdate: 4 December 2025
:sect-event-rule: INSTRUMENTATION POINT, EVENT RULE, AND EVENT
:sect-session: RECORDING SESSION
:sect-domain: TRACING DOMAIN
:sect-channel: CHANNEL AND RING BUFFER
:sect-recording-event-rule: RECORDING EVENT RULE AND EVENT RECORD


NAME
----
lttng-concepts - LTTng concepts


DESCRIPTION
-----------
This manual page documents the concepts of LTTng.

Many other LTTng manual pages refer to this one so that you can
understand what are the various LTTng objects and how they relate to
each other.

The concepts of LTTng{nbsp}{lttng_version} are:

* Instrumentation point, event rule, and event
* Trigger
* Recording session
* Tracing domain
* Channel and ring buffer
* Recording event rule and event record


[[event-rule]]
{sect-event-rule}
-----------------
An _instrumentation point_ is a point, within a piece of software,
which, when executed, creates an LTTng _event_.

LTTng offers various types of instrumentation; see the
``<<inst-point-types,Instrumentation point types>>'' section below to
learn about them.

An _event rule_ is a set of conditions to match a set of events.

When LTTng creates an event{nbsp}__E__, an event rule{nbsp}__ER__ is
said to __match__{nbsp}__E__ when{nbsp}__E__ satisfies *all* the
conditions of{nbsp}__ER__. This concept is similar to a regular
expression which matches a set of strings.

When an event rule matches an event, LTTng _emits_ the event, therefore
attempting to execute one or more actions.

[IMPORTANT]
====
The event creation and emission processes are documentation concepts to
help understand the journey from an instrumentation point to the
execution of actions.

The actual creation of an event can be costly because LTTng needs to
evaluate the arguments of the instrumentation point.

In practice, LTTng implements various optimizations for the Linux kernel
and user space tracing domains (see the ``<<domain,{sect-domain}>>''
section below) to avoid actually creating an event when the tracer
knows, thanks to properties which are independent from the event payload
and current context, that it would never emit such an event. Those
properties are:

* The instrumentation point type (see the
  ``<<inst-point-types,Instrumentation point types>>'' section below).

* The instrumentation point name.

* The instrumentation point log level.

* For a recording event rule (see the
  ``<<recording-event-rule,{sect-recording-event-rule}>>'' section
  below):
** The status of the rule itself.
** The status of the channel (see the ``<<channel,{sect-channel}>>''
   section below).
** The activity of the recording session (started or stopped; see
   the ``<<session,{sect-session}>>'' section below).
** Whether or not the process for which LTTng would create the event is
   allowed to record events (see man:lttng-track(1)).

In other words: if, for a given instrumentation point{nbsp}__IP__, the
LTTng tracer knows that it would never emit an event,
executing{nbsp}__IP__ represents a simple boolean variable check and,
for a Linux kernel recording event rule, a few process attribute checks.
====

As of LTTng{nbsp}{lttng_version}, there are two places where you can
find an event rule:

Recording event rule::
    A specific type of event rule of which the action is to record the
    matched event as an event record.
+
See the ``<<recording-event-rule,{sect-recording-event-rule}>>'' section
below.
+
Create or enable a recording event rule with the
man:lttng-enable-event(1) command.
+
List the recording event rules of a specific recording session
and/or channel with the man:lttng-list(1) and man:lttng-status(1)
commands.

``Event rule matches'' <<trigger,trigger>> condition (since LTTng{nbsp}2.13)::
    When the event rule of the trigger condition matches an event, LTTng
    can execute user-defined actions such as sending an LTTng
    notification, starting a recording session, and more.
+
See man:lttng-add-trigger(1) and man:lttng-event-rule(7).

For LTTng to emit an event{nbsp}__E__,{nbsp}__E__ must satisfy *all* the
basic conditions of an event rule{nbsp}__ER__, that is:

* The instrumentation point from which LTTng creates{nbsp}__E__ has a
  specific type.
+
See the ``<<inst-point-types,Instrumentation point types>>'' section
below.

* A pattern matches the name of{nbsp}__E__ while another pattern
  doesn't.

* The log level of the instrumentation point from which LTTng
  creates{nbsp}__E__ is at least as severe as some value, or is exactly
  some value.

* The fields of the payload of{nbsp}__E__ and the current context fields
  satisfy a filter expression.

A recording event rule has additional, implicit conditions to satisfy.
See the ``<<recording-event-rule,{sect-recording-event-rule}>>'' section
below to learn more.


[[inst-point-types]]
Instrumentation point types
~~~~~~~~~~~~~~~~~~~~~~~~~~~
As of LTTng{nbsp}{lttng_version}, the available instrumentation point
types are, depending on the tracing domain (see the
``<<domain,{sect-domain}>>'' section below):

Linux kernel::
    LTTng tracepoint:::
        A statically defined point in the source code of the kernel
        image or of a kernel module using the LTTng-modules macros.
+
List the available Linux kernel tracepoints with `lttng list --kernel`.
See man:lttng-list(1) to learn more.

    Linux kernel system call:::
        Entry, exit, or both of a Linux kernel system call.
+
List the available Linux kernel system call instrumentation points with
`lttng list --kernel --syscall`. See man:lttng-list(1) to learn more.

    Linux kprobe:::
        A single probe dynamically placed in the compiled kernel code.
+
When you create such an instrumentation point, you set its memory
address or symbol name.

    Linux user space probe:::
        A single probe dynamically placed at the entry of a compiled
        user space application/library function through the kernel.
+
When you create such an instrumentation point, you set:
+
--
With the ELF method::
    Its application/library path and its symbol name.

With the USDT method::
    Its application/library path, its provider name, and its probe name.
+
``USDT'' stands for SystemTap User-level Statically Defined Tracing,
a DTrace-style marker.
--
+
As of LTTng{nbsp}{lttng_version}, LTTng only supports USDT probes which
are :not: reference-counted.

    Linux kretprobe:::
        Entry, exit, or both of a Linux kernel function.
+
When you create such an instrumentation point, you set the memory
address or symbol name of its function.

User space::
    LTTng tracepoint:::
        A statically defined point in the source code of a C/$$C++$$
        application/library using the LTTng-UST macros.
+
List the available Linux kernel tracepoints with
`lttng list --userspace`. See man:lttng-list(1) to learn more.

`java.util.logging`, Apache log4j{nbsp}1.x, Apache Log4j{nbsp}2, and Python::
    Java or Python logging statement:::
        A method call on a Java or Python logger attached to an
        LTTng-UST handler.
+
List the available Java and Python loggers with `lttng list --jul`,
`lttng list --log4j`, `lttng list --log4j2`, and `lttng list --python`.
See man:lttng-list(1) to learn more.


[[trigger]]
TRIGGER
-------
A _trigger_ associates a condition to one or more actions.

When the condition of a trigger is satisfied, LTTng attempts to execute
its actions.

As of LTTng{nbsp}{lttng_version}, the available trigger conditions and
actions are:

Conditions::
+
* The consumed buffer size of a given recording
  session (see the ``<<session,{sect-session}>>'' section below)
  becomes greater than some value.

* The buffer usage of a given channel (see the
  ``<<channel,{sect-channel}>>'' section below) becomes greater than
  some value.

* The buffer usage of a given channel becomes less than some value.

* There's an ongoing recording session rotation (see the
  ``<<rotation,Recording session rotation>>'' section below).

* A recording session rotation becomes completed.

* An event rule matches an event.
+
As of LTTng{nbsp}{lttng_version}, this is the only available condition
when you add a trigger with the man:lttng-add-trigger(1) command. The
other ones are available through the liblttng-ctl C{nbsp}API.

Actions::
+
* Send a notification to a user application.
* Start a given recording session, like man:lttng-start(1) would do.
* Stop a given recording session, like man:lttng-stop(1) would do.
* Archive the current trace chunk of a given recording session (rotate),
  like man:lttng-rotate(1) would do.
* Take a snapshot of a given recording session, like
  man:lttng-snapshot(1) would do.

A trigger belongs to a session daemon (see man:lttng-sessiond(8)), not
to a specific recording session. For a given session daemon, each Unix
user has its own, private triggers. Note, however, that the `root` Unix
user may, for the root session daemon:

* Add a trigger as another Unix user.

* List all the triggers, regardless of their owner.

* Remove a trigger which belongs to another Unix user.

For a given session daemon and Unix user, a trigger has a unique name.

Add a trigger to a session daemon with the man:lttng-add-trigger(1)
command.

List the triggers of your Unix user (or of all users if your
Unix user is `root`) with the man:lttng-list-triggers(1) command.

Remove a trigger with the man:lttng-remove-trigger(1) command.


[[session]]
{sect-session}
--------------
A _recording session_ (named ``tracing session'' prior to
LTTng{nbsp}2.13) is a stateful dialogue between you and a session daemon
(see man:lttng-sessiond(8)) for everything related to event recording.

Everything that you do when you control LTTng tracers to record events
happens within a recording session. In particular, a recording session:

* Has its own name, unique for a given session daemon.

* Has its own set of trace files, if any.

* Has its own state of activity (started or stopped).
+
An active recording session is an implicit recording event rule
condition (see the
``<<recording-event-rule,{sect-recording-event-rule}>>'' section below).

* Has its own mode (local, network streaming, snapshot, or live).
+
See the ``<<session-modes,Recording session modes>>'' section below to
learn more.

* Has its own channels (see the ``<<channel,{sect-channel}>>'' section
  below) to which are attached their own recording event rules.

* Has its own process attribute inclusion sets (see man:lttng-track(1)).

Those attributes and objects are completely isolated between different
recording sessions.

A recording session is like an ATM session: the operations you do on the
banking system through the ATM don't alter the data of other users of
the same system. In the case of the ATM, a session lasts as long as your
bank card is inside. In the case of LTTng, a recording session lasts from
the man:lttng-create(1) command to the man:lttng-destroy(1) command.

A recording session belongs to a session daemon (see
man:lttng-sessiond(8)). For a given session daemon, each Unix user has
its own, private recording sessions. Note, however, that the `root` Unix
user may operate on or destroy another user's recording session.

Create a recording session with the man:lttng-create(1) command.

List the recording sessions of the connected session daemon with
the man:lttng-list(1) command.

Start and stop one or more recording session with the man:lttng-start(1)
and man:lttng-stop(1) commands.

Save and load a recording session with the man:lttng-save(1) and
man:lttng-load(1) commands.

Archive the current trace chunk of (rotate) a recording session with the
man:lttng-rotate(1) command.

Destroy one or more recording sessions with the
man:lttng-destroy(1) command.


Current recording session
~~~~~~~~~~~~~~~~~~~~~~~
When you run the man:lttng-create(1) command, LTTng creates the
`$LTTNG_HOME/.lttngrc` file if it doesn't exist (`$LTTNG_HOME` defaults
to `$HOME`).

`$LTTNG_HOME/.lttngrc` contains the name of the _current recording
session_.

When you create a new recording session with the `create` command, LTTng
updates the current recording session.

The following man:lttng(1) commands select the current recording session
if you don't specify one:

* man:lttng-add-context(1)
* man:lttng-clear(1)
* man:lttng-destroy(1)
* man:lttng-disable-channel(1)
* man:lttng-disable-event(1)
* man:lttng-disable-rotation(1)
* man:lttng-enable-channel(1)
* man:lttng-enable-event(1)
* man:lttng-enable-rotation(1)
* man:lttng-regenerate(1)
* man:lttng-reclaim-memory(1)
* man:lttng-rotate(1)
* man:lttng-save(1)
* man:lttng-snapshot(1)
* man:lttng-start(1)
* man:lttng-status(1)
* man:lttng-stop(1)
* man:lttng-track(1)
* man:lttng-untrack(1)
* man:lttng-view(1)

Set the current recording session manually with the
man:lttng-set-session(1) command, without having to edit the `.lttngrc`
file.


[[session-modes]]
Recording session modes
~~~~~~~~~~~~~~~~~~~~~~~
LTTng offers four recording session modes:

Local mode::
    Write the trace data to the local file system.

Network streaming mode::
    Send the trace data over the network to a listening relay daemon
    (see man:lttng-relayd(8)).

Snapshot mode::
    Only write the trace data to the local file system or send it to a
    listening relay daemon (man:lttng-relayd(8)) when LTTng takes a
    snapshot.
+
LTTng forces all the channels (see the ``<<channel,{sect-channel}>>''
section below) to be created to be configured to be snapshot-ready.
+
LTTng takes a snapshot of such a recording session when any of the
following is true:
+
--
* You run the man:lttng-snapshot(1) command.

* LTTng executes a `snapshot-session` trigger action (see the
  ``<<trigger,TRIGGER>>'' section above).
--

Live mode::
    Send the trace data over the network to a listening relay daemon
    (see man:lttng-relayd(8)) for live reading.
+
An LTTng live reader (for example, man:babeltrace2(1)) can connect to
the same relay daemon to receive trace data while the recording session is
active.


[[rotation]]
Recording session rotation
~~~~~~~~~~~~~~~~~~~~~~~~~~
A _recording session rotation_ is the action of archiving the current
trace chunk of the recording session to the file system.

Once LTTng archives a trace chunk, it does :not: manage it anymore: you
can read it, modify it, move it, or remove it.

An _archived trace chunk_ is a collection of metadata and data stream
files which form a self-contained LTTng trace. See the
``<<trace-chunk-naming,Trace chunk naming>>'' section below to learn how
LTTng names a trace chunk archive directory.

The _current trace chunk_ of a given recording session includes:

* The stream files which LTTng already wrote to the file system, and
  which are not part of a previously archived trace chunk, since the
  most recent event amongst:

** The first time the recording session was started, either with the
   man:lttng-start(1) command or with a `start-session` trigger action
   (see the ``<<trigger,TRIGGER>>'' section above).

** The last rotation, performed with:

*** An man:lttng-rotate(1) command.

*** A rotation schedule previously set with
    man:lttng-enable-rotation(1).

*** An executed `rotate-session` trigger action (see the
    ``<<trigger,TRIGGER>>'' section above).

* The content of all the non-flushed sub-buffers of the channels of the
  recording session.


[[trace-chunk-naming]]
Trace chunk archive naming
~~~~~~~~~~~~~~~~~~~~~~~~~~
A trace chunk archive is a subdirectory of the `archives` subdirectory
within the output directory of a recording session (see the
nloption:--output option of the man:lttng-create(1) command and
of man:lttng-relayd(8)).

A trace chunk archive contains, through tracing domain and possibly
UID/PID subdirectories, metadata and data stream files.

A trace chunk archive is, at the same time:

* A self-contained LTTng trace.

* A member of a set of trace chunk archives which form the complete
  trace of a recording session.

In other words, an LTTng trace reader can read both the recording
session output directory (all the trace chunk archives), or a
single trace chunk archive.

When LTTng performs a recording session rotation, it names the resulting
trace chunk archive as such, relative to the output directory of the
recording session:

[verse]
archives/__BEGIN__-__END__-__ID__

__BEGIN__::
    Date and time of the beginning of the trace chunk archive with
    the ISO{nbsp}8601-compatible __YYYYmmddTHHMMSS±HHMM__ form, where
    __YYYYmmdd__ is the date and __HHMMSS±HHMM__ is the time with the
    time zone offset from UTC.
+
Example: `20171119T152407-0500`

__END__::
    Date and time of the end of the trace chunk archive with
    the ISO{nbsp}8601-compatible __YYYYmmddTHHMMSS±HHMM__ form, where
    __YYYYmmdd__ is the date and __HHMMSS±HHMM__ is the time with the
    time zone offset from UTC.
+
Example: `20180118T152407+0930`

__ID__::
    Unique numeric identifier of the trace chunk within its recording
    session.

Trace chunk archive name example:

----
archives/20171119T152407-0500-20171119T151422-0500-3
----


[[domain]]
{sect-domain}
-------------
A _tracing domain_ identifies a type of LTTng tracer.

A tracing domain has its own properties and features.

There are currently five available tracing domains:

[options="header"]
|===
|Tracing domain |``Event rule matches'' trigger condition option |Option for other CLI commands

|Linux kernel
|nloption:--type option starts with `kernel:`
|nloption:--kernel

|User space
|nloption:--type option starts with `user:`
|nloption:--userspace

|`java.util.logging` (JUL)
|nloption:--type option starts with `jul:`
|nloption:--jul

|Apache log4j{nbsp}1.x
|nloption:--type option starts with `log4j:`
|nloption:--log4j

|Apache Log4j{nbsp}2
|nloption:--type option starts with `log4j2:`
|nloption:--log4j2

|Python
|nloption:--type option starts with `python:`
|nloption:--python
|===

You must specify a tracing domain to target a type of LTTng tracer when
using some man:lttng(1) commands to avoid ambiguity. For example,
because the Linux kernel and user space tracing domains support named
tracepoints as instrumentation points (see the
``<<"event-rule","{sect-event-rule}">>'' section above), you need to
specify a tracing domain when you create an event rule because both
tracing domains could have tracepoints sharing the same name.

You can create channels (see the ``<<channel,{sect-channel}>>'' section
below) in the Linux kernel and user space tracing domains. The other
tracing domains have a single, default channel.


[[channel]]
{sect-channel}
--------------
A _channel_ is an object which is responsible for a set of ring buffers.

Each ring buffer is divided into multiple _sub-buffers_. When a
recording event rule (see the
``<<recording-event-rule,{sect-recording-event-rule}>>'' section below)
matches an event, LTTng can record it to one or more sub-buffers of one
or more channels.

When you create a channel with the man:lttng-enable-channel(1) command,
you set its final attributes, that is:

* Its buffering scheme.
+
See the ``<<channel-buf-scheme,Buffering scheme>>'' section below.

* Its buffer preallocation policy.
+
See the ``<<channel-buf-prealloc,Buffering preallocation policy>>''
section below.

* Its automatic memory reclaim policy.
+
See the ``<<channel-reclaim,Memory reclaim>>'' section below.

* What to do when there's no
  space left for a new event record because all sub-buffers are full.
+
See the ``<<channel-er-loss-mode,Event record loss mode>>'' section
below.

* The size of each ring buffer and how many sub-buffers a ring buffer
  has.
+
See the ``<<channel-sub-buf-size-count,Sub-buffer size and count>>''
section below.

* The size of each trace file LTTng writes for this channel and the
  maximum count of trace files.
+
See the ``<<channel-max-trace-file-size-count,Maximum trace file size
and count>>'' section below.

* The periods of its read, switch, monitor, and watchdog timers.
+
See the ``<<channel-timers,Timers>>'' section below.

* For a Linux kernel channel: its output type (man:mmap(2) or
  man:splice(2)).
+
See the nloption:--output option of the man:lttng-enable-channel(1)
command.

* For a user space channel: the value of its blocking timeout.
+
See the nloption:--blocking-timeout option of the
man:lttng-enable-channel(1) command.

Note that the man:lttng-enable-event(1) command can automatically create
a default channel with sane defaults when no channel exists for the
provided tracing domain.

A channel is always associated to a tracing domain (see the
``<<domain,{sect-domain}>>'' section below). The `java.util.logging`
(JUL), Apache log4j{nbsp}1.x, Apache Log4j{nbsp}2, and Python
tracing domains each have a default channel which you can't configure.

A channel owns recording event rules.

List the channels of a given recording session with the
man:lttng-list(1) and man:lttng-status(1) commands.

Disable an enabled channel with the man:lttng-disable-channel(1)
command.


[[channel-buf-scheme]]
Buffering scheme
~~~~~~~~~~~~~~~~
The buffering scheme of a channel is:

* The **buffer ownership model**: specifies whether the system, each
  Unix user, or each process owns its own ring buffers.

* The **buffer allocation policy**: specifies whether LTTng tracers
  allocate ring buffers per channel or per CPU.

The following list shows how LTTng allocates ring buffers based on the
values of the nloption:--buffer-ownership and
nloption:--buffer-allocation options of the man:lttng-enable-channel(1)
command. For all examples, assume that Unix user `alice` created
the channel `alice-chan` and Unix user `bob` created the channel
`bob-chan`:

nloption:--buffer-ownership=**user** and nloption:--buffer-allocation=**per-cpu**::
+
[verse]
Channel 'alice-channel'
  For the data produced by any process of user 'alice'
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3
Channel 'bob-channel'
  For the data produced by any process of user 'bob'
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3

nloption:--buffer-ownership=**process** and nloption:--buffer-allocation=**per-cpu**::
+
[verse]
Channel `alice-channel`
  For the data produced by process 'my-db' of user 'alice'
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3
  For the data produced by process 'my-app' of user 'alice'
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3
Channel 'bob-channel'
  For the data produced by process 'my-server' of user 'bob'
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3

nloption:--buffer-ownership=**system** and nloption:--buffer-allocation=**per-cpu**::
+
[verse]
Channel 'alice-channel'
  For the data of the Linux kernel
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3
Channel 'bob-channel'
  For the data of the Linux kernel
    Ring buffer for CPU 0
    Ring buffer for CPU 1
    Ring buffer for CPU 2
    Ring buffer for CPU 3

nloption:--buffer-ownership=**user** and nloption:--buffer-allocation=**per-channel**::
+
[verse]
Channel 'alice-channel'
  For the data produced by any process of user 'alice'
    Single ring buffer for all CPUs
Channel 'bob-channel'
  For the data produced by any process of user 'bob'
    Single ring buffer for all CPUs

nloption:--buffer-ownership=**user** and nloption:--buffer-allocation=**per-channel**::
+
[verse]
Channel 'alice-channel'
  For the data produced by process 'my-db' of user 'alice'
    Single ring buffer for all CPUs
  For the data produced by process 'my-app' of user 'alice'
    Single ring buffer for all CPUs
Channel 'bob-channel'
  For the data produced by process 'my-server' of user 'bob'
    Single ring buffer for all CPUs

The per-process buffer ownership model tends to consume more memory than
the per-user option because systems generally have more instrumented
processes than Unix users running instrumented processes. However, the
per-process model ensures that one process having a high event
throughput won't fill all the shared sub-buffers of the same Unix user,
only its own. It also provides complete isolation between processes:
one process cannot read or corrupt the trace data of another.


[[channel-buf-prealloc]]
Buffer preallocation policy
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The buffer preallocation policy of a user space channel specifies
_when_ LTTng allocates the backing memory for the ring buffers
of the channel.

The available buffer preallocation policies are:

Preallocate::
    LTTng allocates the full backing memory for all ring buffers when
    you create the channel.
+
This policy provides consistent performance but uses more memory
upfront. Use this policy to optimize instrumented application
startup time.

On demand::
    LTTng allocates backing memory for ring buffers incrementally, as
    needed during tracing.
+
This policy uses less initial memory but may introduce slight
performance overhead when allocating new memory during tracing.

As of LTTng{nbsp}{lttng_version}, the implicit buffer
preallocation policy of a Linux kernel channel is always to preallocate
backing memory.

Set the buffer preallocation policy of a user space channel with the
nloption:--buffer-preallocation option of the
man:lttng-enable-channel(1) command.


[[channel-reclaim]]
Memory reclaim
~~~~~~~~~~~~~~
A channel memory reclaim operation makes it
possible to free memory which the eligible sub-buffers of a
user space channel use.

The channel memory reclaim operation behaviour depends on the
configured event record loss mode (see the
``<<channel-er-loss-mode,Event record loss mode>>'' section
below) of the channel:

Discard mode::
    Only reclaim sub-buffers that have been both delivered (completed
    and ready to be consumed) _and_
    consumed, since unconsumed data would be lost forever.

Overwrite mode::
    Reclaim any delivered sub-buffer, since old data will be
    overwritten anyway: no need to wait for consumption.

There are two ways to reclaim channel memory:

Immediate reclaim::
    Use the man:lttng-reclaim-memory(1) command.

Automatic reclaim::
    LTTng may automatically reclaim the memory of a
    user space channel.
+
This is more or less equivalent to automatically running the
man:lttng-reclaim-memory(1) command. LTTng automatically evaluates
the eligibility of sub-buffers for a memory reclaim operation at
regular intervals (every half the configured maximal age, but also
at least a period defined at build time) and reclaims sub-buffers
of which the age of the _closing time_ have exceeded the age threshold.
+
Use the nloption:--auto-reclaim-memory-older-than option of
the man:lttng-enable-channel(1) command.

In both cases, you may specify a sub-buffer _age threshold_: only
sub-buffers of which the age of the _closing time_ is older than the
threshold are eligible for a memory reclaim. Moreover, with some
age threshold{nbsp}__T__:

If __T__{nbsp}>{nbsp}last stream activity age::
    The sub-buffers of which the age, at the moment of closing them, is
    greater than{nbsp}__T__ are eligible for a memory reclaim.

If __T__{nbsp}<{nbsp}last stream activity age::
    All the sub-buffers are eligible.
+
The LTTng tracers flush for the targeted channel so that any open
sub-buffer gets closed to become eligible.


[[channel-er-loss-mode]]
Event record loss mode
~~~~~~~~~~~~~~~~~~~~~~
When LTTng emits an event, LTTng can record it to a specific, available
sub-buffer within the ring buffers of specific channels. When there's no
space left in a sub-buffer, the tracer marks it as consumable and
another, available sub-buffer starts receiving the following event
records. An LTTng consumer daemon eventually consumes the marked
sub-buffer, which returns to the available state.

In an ideal world, sub-buffers are consumed faster than they are filled.
In the real world, however, all sub-buffers can be full at some point,
leaving no space to record the following events.

By default, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when
there's no available sub-buffer to record an event, it's acceptable to
lose event records when the alternative would be to cause substantial
delays in the execution of the instrumented application. LTTng
privileges performance over integrity; it aims at perturbing the
instrumented application as little as possible in order to make the
detection of subtle race conditions and rare interrupt cascades
possible.

Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports
a _blocking mode_. See the nloption:--blocking-timeout of the
man:lttng-enable-channel(1) command to learn how to use the blocking
mode.

When it comes to losing event records because there's no available
sub-buffer, or because the blocking timeout of the channel is
reached, the _event record loss mode_ of the channel determines what to
do. The available event record loss modes are:

Discard mode::
    Drop the newest event records until a sub-buffer becomes available.
+
This is the only available mode when you specify a blocking timeout.
+
With this mode, LTTng increments a count of lost event records when an
event record is lost and saves this count to the trace. A trace reader
can use the saved discarded event record count of the trace to decide
whether or not to perform some analysis even if trace data is known to
be missing.

Overwrite mode::
    Clear the sub-buffer containing the oldest event records and start
    writing the newest event records there.
+
This mode is sometimes called _flight recorder mode_ because it's
similar to a https://en.wikipedia.org/wiki/Flight_recorder[flight
recorder]: always keep a fixed amount of the latest data. It's also
similar to the roll mode of an oscilloscope.
+
Since LTTng{nbsp}2.8, with this mode, LTTng writes to a given sub-buffer
its sequence number within its data stream. With a local, network
streaming, or live recording session (see the
``<<session-modes,Recording session modes>>'' section above), a trace
reader can use such sequence numbers to report lost packets. A trace
reader can use the saved discarded sub-buffer (packet) count of the
trace to decide whether or not to perform some analysis even if trace
data is known to be missing.
+
With this mode, LTTng doesn't write to the trace the exact number of
lost event records in the lost sub-buffers.

Which mechanism you should choose depends on your context: prioritize
the newest or the oldest event records in the ring buffer?

Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
as soon as a there's no space left for a new event record, whereas in
discard mode, the tracer only discards the event record that doesn't
fit.

Set the event record loss mode of a channel with the nloption:--discard
and nloption:--overwrite options of the man:lttng-enable-channel(1)
command.

There are a few ways to decrease your probability of losing event
records. The ``<<channel-sub-buf-size-count,Sub-buffer size and
count>>'' section below shows how to fine-tune the sub-buffer size and
count of a channel to virtually stop losing event records, though at the
cost of greater memory usage.


[[channel-sub-buf-size-count]]
Sub-buffer size and count
~~~~~~~~~~~~~~~~~~~~~~~~~
A channel has one or more ring buffer for each CPU of the target system.

See the ``<<channel-buf-scheme,Buffering scheme>>'' section above to
learn how many ring buffers of a given channel are dedicated to each CPU
or to the whole channel depending on its buffering scheme.

Set the size of each sub-buffer the ring buffers of a channel contain
with the nloption:--subbuf-size option of the
man:lttng-enable-channel(1) command.

Set the number of sub-buffers each ring buffer of a channel contains
with the nloption:--num-subbuf option of the man:lttng-enable-channel(1)
command.

Note that LTTng switching the current sub-buffer of a ring buffer
(marking a full one as consumable and switching to an available one for
LTTng to record the next events) introduces noticeable CPU overhead.
Knowing this, the following list presents a few practical situations
along with how to configure the sub-buffer size and count for them:

High event throughput::
    In general, prefer large sub-buffers to lower the risk of losing
    event records.
+
Having larger sub-buffers also ensures a lower sub-buffer switching
frequency (see the ``<<channel-timers,Timers>>'' section below).
+
The sub-buffer count is only meaningful if you create the channel in
overwrite mode (see the ``<<channel-er-loss-mode,Event record loss
mode>>'' section above): in this case, if LTTng overwrites a sub-buffer,
then the other sub-buffers are left unaltered.

Low event throughput::
    In general, prefer smaller sub-buffers since the risk of losing
    event records is low.
+
Because LTTng emits events less frequently, the sub-buffer switching
frequency should remain low and therefore the overhead of the tracer
shouldn't be a problem.

Low memory system::
    If your target system has a low memory limit, prefer fewer first,
    then smaller sub-buffers.
+
Even if the system is limited in memory, you want to keep the
sub-buffers as large as possible to avoid a high sub-buffer switching
frequency.

Note that LTTng uses https://diamon.org/ctf/[CTF] as its trace format
(see the nloption:--trace-format option of the man:lttng-create(1)
command),
which means event record data is very compact. For example, the average
LTTng kernel event record weights about 32{nbsp}bytes. Therefore, a
sub-buffer size of 1{nbsp}MiB is considered large.

The previous scenarios highlight the major trade-off between a few large
sub-buffers and more, smaller sub-buffers: sub-buffer switching
frequency vs. how many event records are lost in overwrite mode.
Assuming a constant event throughput and using the overwrite mode, the
two following configurations have the same ring buffer total size:

Two sub-buffers of 4{nbsp}MiB each::
    Expect a very low sub-buffer switching frequency, but if LTTng
    ever needs to overwrite a sub-buffer, half of the event records so
    far (4{nbsp}MiB) are definitely lost.

Eight sub-buffers of 1{nbsp}MiB each::
    Expect four times the tracer overhead of the configuration above,
    but if LTTng needs to overwrite a sub-buffer, only the eighth of
    event records so far (1{nbsp}MiB) are definitely lost.

In discard mode, the sub-buffer count parameter is pointless: use two
sub-buffers and set their size according to your requirements.


[[channel-max-trace-file-size-count]]
Maximum trace file size and count
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
By default, trace files can grow as large as needed.

Set the maximum size of each trace file that LTTng writes of a given
channel with the nloption:--tracefile-size option of the man:lttng-enable-channel(1)
command.

When the size of a trace file reaches the fixed maximum size of the
channel, LTTng creates another file to contain the next event records.
LTTng appends a file count to each trace file name in this case.

If you set the trace file size attribute when you create a channel, the
maximum number of trace files that LTTng creates is _unlimited_ by
default. To limit them, use the nloption:--tracefile-count option of
man:lttng-enable-channel(1). When the number of trace files reaches the
fixed maximum count of the channel, LTTng overwrites the oldest trace
file. This mechanism is called _trace file rotation_.

[IMPORTANT]
====
Even if you don't limit the trace file count, always assume that LTTng
manages all the trace files of the recording session.

In other words, there's no safe way to know if LTTng still holds a given
trace file open with the trace file rotation feature.

The only way to obtain an unmanaged, self-contained LTTng trace before
you destroy the recording session is with the recording session rotation
feature (see the ``<<rotation,Recording session rotation>>'' section
above), which is available since LTTng{nbsp}2.11.
====


[[channel-timers]]
Timers
~~~~~~
Each channel can have up to three optional timers:

Switch timer::
    When this timer expires, a sub-buffer switch happens: for each ring
    buffer of the channel, LTTng marks the current sub-buffer as
    consumable and switches to an available one to record the next
    events.
+
A switch timer is useful to ensure that LTTng consumes and commits trace
data to trace files or to a distant relay daemon (man:lttng-relayd(8))
periodically in case of a low event throughput.
+
Such a timer is also convenient when you use large sub-buffers (see the
``<<channel-sub-buf-size-count,Sub-buffer size and count>>'' section
above) to cope with a sporadic high event throughput, even if the
throughput is otherwise low.
+
Set the period of the switch timer of a channel, or disable the timer
altogether, with the nloption:--switch-timer option of the
man:lttng-enable-channel(1) command.

Read timer::
    When this timer expires, LTTng checks for full, consumable
    sub-buffers.
+
By default, the LTTng tracers use an asynchronous message mechanism to
signal a full sub-buffer so that a consumer daemon can consume it.
+
When such messages must be avoided, for example in real-time
applications, use this timer instead.
+
Set the period of the read timer of a channel, or disable the timer
altogether, with the nloption:--read-timer option of the
man:lttng-enable-channel(1) command.

Monitor timer::
    When this timer expires, the consumer daemon samples some channel
    statistics to evaluate the following trigger conditions:
+
--
. The consumed buffer size of a given recording session becomes greater
  than some value.
. The buffer usage of a given channel becomes greater than some value.
. The buffer usage of a given channel becomes less than some value.
--
+
If you disable the monitor timer of a channel{nbsp}__C__:
+
--
* The consumed buffer size value of the recording session of{nbsp}__C__
  could be wrong for trigger condition type{nbsp}1: the consumed buffer
  size of{nbsp}__C__ won't be part of the grand total.

* The buffer usage trigger conditions (types{nbsp}2 and{nbsp}3)
  for{nbsp}__C__ will never be satisfied.
--
+
See the ``<<trigger,TRIGGER>>'' section above to learn more about
triggers.
+
Set the period of the monitor timer of a channel, or disable the timer
altogether, with the nloption:--monitor-timer option of the
man:lttng-enable-channel(1) command.

Watchdog timer::
    When this timer expires, LTTng performs health checks that detect
    recoverable issues and trigger corrective actions to keep trace
    recording running smoothly for the channel.
+
As of LTTng{nbsp}{lttng_version}, the only health check
is a sub-buffer stall detection: with a user space channel having a
per-user ownership model (see the
<<channel-buf-scheme,Buffering scheme>> section above),
an instrumented application writing to a sub-buffer may crash and
make said sub-buffer ``stalled'', that is, never released and
unusable. This can reduce the effective capacity of the channel or
even halt the consumption altogether. When the watchdog timer of
a channel expires and LTTng detects this condition, it marks
the stalled sub-buffer consumable/readable again.
+
Set the period of the watchdog timer of a channel, or disable the timer
altogether, with the nloption:--watchdog-timer option of the
man:lttng-enable-channel(1) command.


[[recording-event-rule]]
{sect-recording-event-rule}
---------------------------
A _recording event rule_ is a specific type of event rule (see the
``<<"event-rule","{sect-event-rule}">>'' section above) of which the
action is to serialize and record the matched event as an _event
record_.

Set the explicit conditions of a recording event rule when you create it
with the man:lttng-enable-event(1) command. A recording event rule also
has the following implicit conditions:

* The recording event rule itself is enabled.
+
A recording event rule is enabled on creation.

* The channel to which the recording event rule is attached is enabled.
+
A channel is enabled on creation.
+
See the ``<<channel,{sect-channel}>>'' section above.

* The recording session of the recording event rule is active (started).
+
A recording session is inactive (stopped) on creation.
+
See the ``<<session,{sect-session}>>'' section above.

* The process for which LTTng creates an event to match is allowed to
  record events.
+
All processes are allowed to record events on recording session
creation.
+
Use the man:lttng-track(1) and man:lttng-untrack(1) commands to select
which processes are allowed to record events based on specific process
attributes.

You always attach a recording event rule to a channel, which belongs to
a recording session, when you create it.

When a recording event rule{nbsp}__ER__ matches an event{nbsp}__E__,
LTTng attempts to serialize and record{nbsp}__E__ to one of the
available sub-buffers of the channel to which{nbsp}__E__ is attached.

When multiple matching recording event rules are attached to the same
channel, LTTng attempts to serialize and record the matched event
_once_. In the following example, the second recording event rule is
redundant when both are enabled:

[role="term"]
----
$ lttng enable-event --userspace hello:world
$ lttng enable-event --userspace hello:world --loglevel=INFO
----

List the recording event rules of a specific recording session
and/or channel with the man:lttng-list(1) and man:lttng-status(1)
commands.

Disable a recording event rule with the man:lttng-disable-event(1)
command.

As of LTTng{nbsp}{lttng_version}, you cannot remove a recording event
rule: it exists as long as its recording session exists.


include::common-footer.txt[]


SEE ALSO
--------
man:lttng(1),
man:lttng-relayd(8),
man:lttng-sessiond(8)
