linux-kernel - Re: [PATCH 10/10] perf record: Add an option to force per-cpu mmaps

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Date:	Fri, 15 Nov 2013 12:56:29 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Mike Galbraith <efault@....de>,
	Namhyung Kim <namhyung@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH 10/10] perf record: Add an option to force per-cpu mmaps


* Adrian Hunter <adrian.hunter@...el.com> wrote:

> On 15/11/13 13:10, Ingo Molnar wrote:
> > 
> > * Adrian Hunter <adrian.hunter@...el.com> wrote:
> > 
> >>> And I also raised why this shouldn't be the default event tracing 
> >>> method instead of a weird config option. Per-cpu tracing is cache 
> >>> compact, it is easier to size properly and in general it is pretty 
> >>> easy to think about. (It also has less of the TSC timestamp 
> >>> ordering problems as per thread tracing, at least in theory.)
> >>>
> >>> Is there something that makes per cpu tracing undesirable as the 
> >>> default?
> >>
> >> One reason is to avoid changing the meaning of existing options.
> > 
> > Well, the way the tracing buffers are set up is a mostly tool internal 
> > matter so in that sense it should be just fine to change the default 
> > behavior - as long as output remains unchanged (which it should).
> > 
> > Or is there any material change in behavior somewhere?
> 
> Inheritance is enabled automatically with per-cpu mmaps,
> although that is one of the reasons people want
> per-cpu mmaps.

So, here's the current status quo, there's 4 basic types of profiling 
that 99% of the people are using, in order of popularity:

	perf record <cmd>
	perf record -a sleep N
	perf record -p <PID>
	perf record -t <TID>

The first two (which I'd guess comprise about 95% of real-world usage) 
have inheritance enabled.

The last two (-p/-t) have inheritance disabled by default.

Correct?

Thanks,

	Ingo
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