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 12:10:32 +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:

> > 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?

> To flip it around, ignore the patches above and apply:

> Subject: [PATCH] perf record: Make per-cpu mmaps the default.

Yay!

> +--per-thread::
> +Use per-thread mmaps.  By default per-cpu mmaps are created.  This option
> +overrides that and uses per-thread mmaps.  A side-effect of that is that
> +inheritance is automatically disabled.  --per-thread is ignored with a warning
> +if combined with -a or -C options.

I think this is the natural thing to do, --per-thread is the 'somewhat 
weird' option that cannot be used in all modes.

Acked-by: Ingo Molnar <mingo@...nel.org>

:-)

Thanks,

	Ingo
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