lttng-concepts(7) ================= :revdate: 3 May 2021 :sect-event-rule: INSTRUMENTATION POINT, EVENT RULE, AND EVENT :sect-session: TRACING 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 * Tracing 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 <> 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 <> 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 <> section below). * The instrumentation point name. * The instrumentation point log level. * For a recording event rule (see the <> section below): ** The status of the rule itself. ** The status of the channel (see the <> section below). ** The activity of the tracing session (started or stopped; see the <> 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 <> 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 tracing session and/or channel with the man:lttng-list(1) and man:lttng-status(1) commands. ``Event rule matches'' <> 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 tracing 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 <> 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 <> 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 <> 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, 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`, 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 tracing session (see the <> section below) becomes greater than some value. * The buffer usage of a given channel (see the <> section below) becomes greater than some value. * The buffer usage of a given channel becomes less than some value. * There's an ongoing tracing session rotation (see the <> section below). * A tracing 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 tracing session, like man:lttng-start(1) would do. * Stop a given tracing session, like man:lttng-stop(1) would do. * Archive the current trace chunk of a given tracing session (rotate), like man:lttng-rotate(1) would do. * Take a snapshot of a given tracing session, like man:lttng-snapshot(1) would do. A trigger belongs to a session daemon (see man:lttng-sessiond(8)), not to a specific tracing 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 _tracing session_ 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 tracing session. In particular, a tracing 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 tracing session is an implicit recording event rule condition (see the <> section below). * Has its own mode (local, network streaming, snapshot, or live). + See the <> section below to learn more. * Has its own channels (see the <> 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 tracing sessions. A tracing 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 tracing session lasts from the man:lttng-create(1) command to the man:lttng-destroy(1) command. A tracing session belongs to a session daemon (see man:lttng-sessiond(8)). For a given session daemon, each Unix user has its own, private tracing sessions. Note, however, that the `root` Unix user may operate on or destroy another user's tracing session. Create a tracing session with the man:lttng-create(1) command. List the tracing sessions of the connected session daemon with the man:lttng-list(1) command. Start and stop a tracing session with the man:lttng-start(1) and man:lttng-stop(1) commands. Save and load a tracing session with the man:lttng-save(1) and man:lttng-load(1) commands. Archive the current trace chunk of (rotate) a tracing session with the man:lttng-rotate(1) command. Destroy a tracing session with the man:lttng-destroy(1) command. Current tracing 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 tracing session_. When you create a new tracing session with the `create` command, LTTng updates the current tracing session. The following man:lttng(1) commands select the current tracing 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-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 tracing session manually with the man:lttng-set-session(1) command, without having to edit the `.lttngrc` file. [[session-modes]] Tracing session modes ~~~~~~~~~~~~~~~~~~~~~ LTTng offers four tracing 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 <> section below) to be created to be configured to be snapshot-ready. + LTTng takes a snapshot of such a tracing session when: + -- * You run the man:lttng-snapshot(1) command. * LTTng executes a `snapshot-session` trigger action (see the <> 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 tracing session is active. [[rotation]] Tracing session rotation ~~~~~~~~~~~~~~~~~~~~~~~~ A _tracing session rotation_ is the action of archiving the current trace chunk of the tracing 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 <> section below to learn how LTTng names a trace chunk archive directory. The _current trace chunk_ of a given tracing 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 tracing session was started, either with the man:lttng-start(1) command or with a `start-session` trigger action (see the <> 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 <> section above). * The content of all the non-flushed sub-buffers of the channels of the tracing 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 tracing 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 tracing session. In other words, an LTTng trace reader can read both the tracing session output directory (all the trace chunk archives), or a single trace chunk archive. When LTTng performs a tracing session rotation, it names the resulting trace chunk archive as such, relative to the output directory of the tracing 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 tracing 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:--domain=++kernel++ |nloption:--kernel |User space |nloption:--domain=++user++ |nloption:--userspace |`java.util.logging` (JUL) |nloption:--domain=++jul++ |nloption:--jul |Apache log4j |nloption:--domain=++log4j++ |nloption:--log4j |Python |nloption:--domain=++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 <> 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 <> section below. * What to do when there's no space left for a new event record because all sub-buffers are full. + See the <> section below. * The size of each ring buffer and how many sub-buffers a ring buffer has. + See the <> section below. * The size of each trace file LTTng writes for this channel and the maximum count of trace files. + See the <> section below. * The periods of its read, switch, and monitor timers. + See the <> 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 <> section below). The `java.util.logging` (JUL), log4j, 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 tracing 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 ~~~~~~~~~~~~~~~~ A channel has at least one ring buffer per CPU. LTTng always records an event to the ring buffer dedicated to the CPU which emits it. The buffering scheme of a user space channel determines what has its own set of per-CPU ring buffers: Per-user buffering (nloption:--buffers-uid option of the man:lttng-enable-channel(1) command):: Allocate one set of ring buffers (one per CPU) shared by all the instrumented processes of: If your Unix user is `root`::: Each Unix user. Otherwise::: Your Unix user. Per-process buffering (nloption:--buffers-pid option of the man:lttng-enable-channel(1) command):: Allocate one set of ring buffers (one per CPU) for each instrumented process of: If your Unix user is `root`::: All Unix users. Otherwise::: Your Unix user. The per-process buffering scheme 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 buffering scheme 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. The buffering scheme of a Linux kernel channel is always to allocate a single set of ring buffers for the whole system. This scheme is similar to the per-user option, but with a single, global user ``running'' the kernel. [[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 tracing session (see the <> 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 <> 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 <> section above to learn how many ring buffers of a given channel are dedicated to each CPU 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 <> section below). + The sub-buffer count is only meaningful if you create the channel in overwrite mode (see the <> 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, 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 tracing 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 tracing session is with the tracing session rotation feature (see the <> 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 <> 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 tracing 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 tracing 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 <> 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. [[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 <> section above. * The tracing session of the recording event rule is active (started). + A tracing session is inactive (stopped) on creation. + See the <> 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 tracing 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 tracing 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 tracing 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 tracing session exists. include::common-footer.txt[] SEE ALSO -------- man:lttng(1), man:lttng-relayd(8), man:lttng-sessiond(8)