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Date: Fri, 15 Nov 2013 13:05:25 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        linux-kernel@vger.kernel.org, David Ahern <dsahern@gmail.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@redhat.com>,
        Mike Galbraith <efault@gmx.de>, Namhyung Kim <namhyung@gmail.com>,
        Paul Mackerras <paulus@samba.org>,
        Stephane Eranian <eranian@google.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>
Subject: Re: [PATCH 10/10] perf record: Add an option to force per-cpu mmaps
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* Peter Zijlstra <peterz@infradead.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