linux-kernel - Re: [PATCH 4/5] perf record: mmap output file

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Message-ID: <20131119161402.GA20224@gmail.com>
Date:	Tue, 19 Nov 2013 17:14:02 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org, jolsa@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5


* David Ahern <dsahern@...il.com> wrote:

> > Well, we could exclude the profiling task itself from profiling 
> > events (just like ftrace and core bits of perf does it out of 
> > necessity), but I intentionally wanted to avoid that, to make sure 
> > we are honest and to make sure people don't tolerate profiling 
> > overhead that disturbs other workloads.
> 
> Samples generated by perf itself need to be observable -- e.g. 
> process scheduling I want to see the time consumed by the data 
> collector itself and there are times when 'perf trace -- perf ...' 
> is useful.

Absolutely agreed - a measurement instrument affects the measurement, 
and we must not try to hide that.

Still we can try to make the disturbance smaller and more managable.

For example if I have enough RAM it should be possible to run perf 
record with a 1 GB ring-buffer, and in that case as long as the 
perf.data is smaller than 1 GB there should be no writeout or any 
other IO activity until the measurement ends.

Thanks,

	Ingo
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