[lttng-dev] delay between trigger and action

[Yitschak, Yehuda]

Hi everyone

I am experimenting with enabling trace for a specific iteration of a loop in my application.
I created 2 trace points at the start and end of the loop which log the iteration number.
the loop itself creates thousands of trace points. It runs for ~5ms.

on lttng side I created 2 triggers to start and stop my session in case the loop tracepoint registers a specific iteration number.
the triggers seem to work since there is another session entry created under the session folder but there are very few tracepoints.

is it possible that the delay between the event happening and the session trace starting causes loss of events ?
If yes, is there a way around this  ?

here is my setup :

#lttng-status
Recording session session: [inactive]
    Trace output: /home/user/lttng-traces/session-20230523-113537

=== Domain: User space ===

Buffering scheme: per-user

Tracked process attributes
  Virtual process IDs:  all
  Virtual user IDs:     all
  Virtual group IDs:    all

Channels:
-------------
- user-channel: [enabled]

    Attributes:
      Event-loss mode:  discard
      Sub-buffer size:  16777216 bytes
      Sub-buffer count: 128
      Switch timer:     inactive
      Read timer:       inactive
      Monitor timer:    1000000 us
      Blocking timeout: 0 us
      Trace file count: 1 per stream
      Trace file size:  unlimited
      Output mode:      mmap

    Statistics:
      Discarded events: 0

    Recording event rules:
      trace_events* (type: tracepoint) [enabled]
      loop* (type: tracepoint) [enabled]

#lttng list-triggers

- name: iter-start
  owner uid: 1000
  condition: event rule matches
    rule: loop:iteration (type: user tracepoint, filter: iter==2 && start == 1)
    errors: none
  actions:
    start session `session`
      errors: none
  errors: none
- name: iter-stop
  owner uid: 1000
  condition: event rule matches
    rule: loop:iteration (type: user tracepoint, filter: iter==2 && start == 0)
    errors: none
  actions:
    stop session `session`
      errors: none
  errors: none


Thanks

Yehuda
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