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

Thibault, Daniel Daniel.Thibault at drdc-rddc.gc.ca
Wed Apr 16 15:12:27 EDT 2014

>> 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?

Daniel U. Thibault
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