[lttng-dev] memory usage depending on number and size of subbuffers

[Mathieu Desnoyers]

----- Original Message -----
> From: "Daniel Thibault" <Daniel.Thibault at drdc-rddc.gc.ca>
> To: lttng-dev at lists.lttng.org
> Cc: "mathieu desnoyers (mathieu.desnoyers at efficios.com)" <mathieu.desnoyers at efficios.com>
> Sent: Wednesday, April 16, 2014 3:12:27 PM
> Subject: RE: [lttng-dev] memory usage depending on number and size of subbuffers
> 
> ----------------------------------------------------------------------
> >> Date: Wed, 16 Apr 2014 18:22:42 +0200
> >> From: S?bastien Barth?l?my <barthelemy at crans.org>
> >>
> >> LTTng uses more memory than I expected. On the simple experiment below one
> >> can see that the "cached" memory increases by
> >>
> >> subbuf_size*num_cpu*(num_subbuf+1)
> >>
> >> That's not a big deal, but I did not expect the "+1".
> >>
> >> Does somebody know where it comes from?
> 
> > Date: Wed, 16 Apr 2014 17:32:39 +0000 (UTC)
> > From: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> > It is the extra sub-buffer needed for reading a sub-buffer while the
> > other sub-buffers are written into. It's needed for snapshotting
> > data while tracing is active.
> >
> > Mathieu
> 
>    If I'm not mistaken, the "passing sub-buffer" is used in overwrite mode
>    (as opposed to discard mode), not especially for snapshot mode.  It is in
>    fact possible to run in snapshot+discard mode, although that may not be
>    very useful in most use cases.  Can you confirm, Mathieu?

While theoretically possible, I'm not even sure if we allow it, because
it's pretty much useless. But I suspect one could indeed force a channel in
discard mode for a snapshot session. David, can you confirm ?

Thanks,

Mathieu


> 
> Daniel U. Thibault
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com