linux-kernel - Re: [PATCH 17/19] perf tools: Always force PERF_RECORD_FINISHED

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Date:	Fri, 25 Jul 2014 11:14:13 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Jean Pihet <jean.pihet@...aro.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 17/19] perf tools: Always force
 PERF_RECORD_FINISHED_ROUND event

Em Fri, Jul 25, 2014 at 01:34:26PM +0200, Jiri Olsa escreveu:
> On Thu, Jul 24, 2014 at 06:34:51PM -0300, Arnaldo Carvalho de Melo wrote:
> > I think both changes are OK, but should be split in different patches,
 
> right, I'll split it

Thanks!
 
> > [root@zoo /]# perf stat -r 5 perf report --no-ordered-samples > /dev/null
> >    101,171,572,553      instructions              #    1.10  insns per cycle        
> >       30.249514999 seconds time elapsed                                          ( +-  0.48% )

> > [root@zoo /]# perf stat -r 5 perf report --ordered-samples > /dev/null
> >    105,982,144,263      instructions              #    1.04  insns per cycle        
> >       32.636483981 seconds time elapsed                                          ( +-  0.41% )

> so those 2 extra seconds is the ordering time, right? sounds ok

Yeah, but I think its worth investigating if using it is a strict
requirement in all cases, i.e. is it possible to receive out of order
events when sampling on a single CPU? Or a single CPU socket with a
coherent time source? etc.

Providing a way to disable this ordering to be used in corner cases
where it is not a strict requirement and the volume of samples is so
high that reducing processing time like shown above seems to be a
sensible thing to do.

We're in the business of optimizing stuff, huh? :-)
 
- Arnaldo
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