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 13:05:25 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.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


* Peter Zijlstra <peterz@...radead.org> wrote:

> 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.

Correct.

> 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.

Do the first variant, 'perf record <cmd>', already use per CPU 
buffers?

Thanks,

	Ingo
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