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 13:03:29 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	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>,
	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

On Fri, Nov 15, 2013 at 12:56:29PM +0100, Ingo Molnar wrote:
> 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.

Yes, and I would expect it to be disabled for the TID option as you
explicitly select a single threads.

For the process wide thing it would make sense to enable inheritance by
default though.

So the big trade-off is that for single threaded processes which do not
fork you now have a single buffer, whereas with the inheritance option
you'll end up with nr_cpus buffers by default.

I suppose for most normal people that's not really an issue; and I
suppose all people with silly large machines already pay extra attention
-- but at least make it explicit and very clear that this is so.


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