linux-kernel - Re: [PATCH 1/1] perf, tool: partial callgrap and time support in perf record

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Message-ID: <20150316204826.GK31334@tassilo.jf.intel.com>
Date:	Mon, 16 Mar 2015 13:48:26 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	"Liang, Kan" <kan.liang@...el.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"eranian@...gle.com" <eranian@...gle.com>
Subject: Re: [PATCH 1/1] perf, tool: partial callgrap and time support in
 perf record

On Mon, Mar 16, 2015 at 08:35:30PM +0000, Liang, Kan wrote:
> 
> 
> > 
> > Hi Kan,
> > 
> > On Fri, Mar 13, 2015 at 02:18:07AM +0000, kan.liang@...el.com wrote:
> > > From: Kan Liang <kan.liang@...el.com>
> > >
> > > When multiple events are sampled it may not be needed to collect
> > > callgraphs for all of them. The sample sites are usually nearby, and
> > > it's enough to collect the callgraphs on a reference event (such as
> > > precise cycles or precise instructions). Similarly we also don't need
> > > fine grained time stamps on all events, as it's enough to have time
> > > stamps on the regular reference events. This patchkit adds the ability
> > > to turn off callgraphs and time stamps per event. This in term can
> > > reduce sampling overhead and the size of the perf.data (add some data)
> > 
> > Have you taken a look into group sampling feature?
> > (e.g. perf record -e '{ev1,ev2}:S')
> > 
> 
> I didn't find any issues when running group read. 
> The patch doesn't change the behavior of group read features.
> 
> Did you observe any issues after applying the patch?

I think Namhyungs questions was if group read can be used
instead to decrease the data size.

The answer is no: it solves a different problem. Group read 
is just fine granuality counting. It cannot be used
to sample for multiple events in parallel.

-Andi
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