[lttng-dev] Working with lttng enable-channel

[David Goulet]

There is indeed something wrong.

Avi Goren:
> Hi,
> 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:
> -------------------------------------------------------------------------
> root at ubuntu:~# lttng list
> Currently no available tracing session
> root at ubuntu:~# lttng create ses
> Session ses created.
> Traces will be written in /home/avig/lttng-traces/ses-20130624-163340
> root at ubuntu:~# lttng enable-event -a -u -s ses -c ch
> All UST events are enabled in channel ch

This command creates and enables all event in the channel "ch" with the default
settings for the channel.

> root at ubuntu:~# lttng disable-channel ch -u -s ses
> UST channel ch disabled for session ses
> root at ubuntu:~# lttng enable-channel ch -u --subbuf-size 1024 --num-subbuf 1
> UST channel ch enabled for session ses

This command actually only enable the *already* existing channel. Once a channel
is created, you can't change the attributes, it's immutable.

> root at ubuntu:~# 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]
> 
> root at ubuntu:~# 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?

If you want to control the flushing timing of a channel, use "--switch-timer".
For instance, for a flush buffer at each 200ms:

$ lttng enable-channel ch -u --switch timer 200000

Hope this can help!
David

> Thanks,
> Avi
> 
> 
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