[lttng-dev] Working with lttng enable-channel

[Thibault, Daniel]

-----Message d'origine-----
Message: 7
Date: Mon, 24 Jun 2013 17:28:26 +0300
From: Avi Goren <avi at reduxio.com>

I'm using lttng-ust and in the process of the attempt to gain some flush
frequency control I encountered a problem.
I'm having difficulties with the use of enable-channel.
it doesn't seem to apply property changes. here's an example:
-------------------------------------------------------------------------
# lttng create ses
Session ses created.
Traces will be written in /home/avig/lttng-traces/ses-20130624-163340
# lttng enable-event -a -u -s ses -c ch
All UST events are enabled in channel ch
# lttng disable-channel ch -u -s ses
UST channel ch disabled for session ses
# lttng enable-channel ch -u --subbuf-size 1024 --num-subbuf 1
UST channel ch enabled for session ses
# lttng list ses -c ch
Tracing session ses: [inactive]
    Trace path: /home/avig/lttng-traces/ses-20130624-163340

=== Domain: UST global ===

Buffer type: per PID

- ch: [enabled]

    Attributes:
      write mode: discard
      subbufers size: 4096 bytes
      number of subbufers: 4
      switch timer interval: 0 usec
      read timer interval: 0 usec
      output: mmap()

    Events:
      * (type: tracepoint) [enabled]

# lttng version
lttng version 2.2.0-rc2 - Cuda
...
-------------------------------------------------------------------------
Am I doing something wrong?
Is there a way to control flushing policy besides the use of enable-channel?
Thanks,
Avi
-----Fin du message d'origine-----

   From my dabbling with the buffers-pid and buffers-uid options, I strongly suspect you need to enable the channel with its subbuf-size and num-subbuf options *before* using enable-event, otherwise enable-event creates the channel with a default configuration as a side effect.

Daniel U. Thibault
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