[ltt-dev] limiting LTTng to a certain set of cores

[Andrew McDermott]

> * Andrew McDermott (andrew.mcdermott at windriver.com) wrote:
>> 
>> Is there an easy way to limit LTTng to trace a specified set of cores
>> only?  I ask because in some 64+ core systems the data set collected
>> becomes too large for analysis and the tasks to be analyzed are always
>> bound to certain cores (using affinity).
>> 
>> I'm anticipating that this would require changes to LTTng - what I would
>> like to know is how invasive those changes would need to be.
>> 
>
> Hi Andrew,
>
> Basically, this could be done on a per tracing-session/per-channel basis
> by adding an "active core" bit mask to test within ltt_vtrace and
> ltt_serialize_data, in the loop where it iterates on active traces and
> calls the "filter" callback. This bitmask would sit within the channel
> structure. Setting the active cores should be added to the debugfs
> interface :

Thanks for the info!

Perhaps it is something to consider for a future release.

One of the issues I'm facing is that the trace is large and by large I
mean I have to transfer the trace in its entirety from a remote
location.  If I could collect on a subset of the cores then I would have
less to transfer (over my very slow link).

>
> People would do :
> echo tracename > /mnt/debugfs/ltt/setup_trace
> echo CPUNR > /mnt/debugfs/ltt/control/tracename/channel/channelname/cpus
> cat /mnt/debugfs/ltt/control/tracename/channel/channelname/cpus
> ...
>
> Maybe we should replace the "enable" test by a cpu mask test with all
> cpus active to save some cycles...
>
> And I don't particularly care about the exact debugfs file interface
> (file name, the way the cpus are listed). Ideally if we can find
> somewhere else in the Linux kernel where it is already done, that would
> be a great inspiration source.
>
> Thanks,
>
> Mathieu
>
>> Thanks,
>> Andy.
>> 
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-- 
andy