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

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Message-ID: <20150318071406.GS943@sejong>
Date:	Wed, 18 Mar 2015 16:14:06 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	"Liang, Kan" <kan.liang@...el.com>,
	"acme@...nel.org" <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"eranian@...gle.com" <eranian@...gle.com>, jolsa@...nel.org
Subject: Re: [PATCH 1/1] perf, tool: partial callgrap and time support in
 perf record

Hi Andi,

(Add Jiri to CC)

On Mon, Mar 16, 2015 at 01:48:26PM -0700, Andi Kleen wrote:
> On Mon, Mar 16, 2015 at 08:35:30PM +0000, Liang, Kan wrote:
> > > 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.

Right!


> 
> 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.

But group read disables sampling for non-leader events so the number
of total samples should be small, no?

Thanks,
Namhyung
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