.nf
Enable tracing channel
-To enable event, you must first enable a channel which contains event(s).
+To enable an event, you must enable both the event and the channel that
+contains it.
If \fB\-s, \-\-session\fP is omitted, the session name is taken from the .lttngrc
file.
Flight recorder mode : overwrites events when subbuffers are full
\-\-subbuf-size SIZE
Subbuffer size in bytes (default: 4096, kernel default: 262144)
+ Needs to be a power of 2 for both tracers
\-\-num-subbuf NUM
Number of subbuffers (default: 4)
- Needs to be a power of 2 for kernel and ust tracers
+ Needs to be a power of 2 for both tracers
\-\-switch-timer USEC
- Switch subbuffer timer interval in usec (default: 0)
- Needs to be a power of 2 for kernel and ust tracers
+ Switch subbuffer timer interval in µsec (default: 0)
\-\-read-timer USEC
- Read timer interval in usec (default: 200)
+ Read timer interval in µsec (default: 200)
\-\-output TYPE
Channel output type. Possible values: mmap, splice
\-\-buffers-uid
\-\-filter 'expression'
Set a filter on a newly enabled event. Filter expression on event
- fields, event recording depends on evaluation. Only specify on first
- activation of a given event within a session. Filter only allowed when
- enabling events within a session before tracing is started. If the
- filter fails to link with the event within the traced domain, the event
- will be discarded. Currently, filter is only implemented for the
- user-space tracer.
+ fields and context. Event recording depends on evaluation. Only
+ specify on first activation of a given event within a session.
+ Filter only allowed when enabling events within a session before
+ tracing is started. If the filter fails to link with the event
+ within the traced domain, the event will be discarded.
+ Currently, filter is only implemented for the user-space tracer.
Expression examples:
Wildcards are allowed at the end of strings:
'seqfield1 == "te*"'
In string literals, the escape character is a '\\'. Use '\\*' for
- the '*' character, and '\\\\' for the '\\' character.
+ the '*' character, and '\\\\' for the '\\' character. Wildcard
+ match any sequence of characters, including an empty sub-string
+ (match 0 or more characters).
+
+ Context information can be used for filtering. The examples
+ below show usage of context filtering on process name (with a
+ wildcard), process ID range, and unique thread ID for filtering.
+ The process and thread ID of running applications can be found
+ under columns "PID" and "LWP" of the "ps -eLf" command.
+
+ '$ctx.procname == "demo*"'
+ '$ctx.vpid >= 4433 && $ctx.vpid < 4455'
+ '$ctx.vtid == 1234'
.fi
.IP "\fBdisable-channel\fP NAME[,NAME2,...] [\-k|\-u] [OPTIONS]"