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

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Message-ID: <525DF2CA.1060102@gmail.com>
Date:	Tue, 15 Oct 2013 19:58:34 -0600
From:	David Ahern <dsahern@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
CC:	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>, Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf record: mmap output file - v2

On 10/15/13 7:52 PM, Namhyung Kim wrote:
> Aha, okay.  So it mostly matters to syscall tracing, right?  For a
> normal record session, it seems that the effect is not that large:

Yes, that's in the description "When recording raw_syscalls for the 
entire system"

There is a small benefit to all record sessions -- mmap+memcpy has less 
overhead than write(). I still need to look at Ingo's suggestion to use 
non-temporal stores which might reduce the overhead of the memcpy.

Try a workload that generates a HUGE data file -- say a full kernel 
build (e.g., perf record -g -- make O=/tmp/kbuild -j 16). You should see 
a much larger benefit from the mmap route. Be sure to use your callchain 
enhancements to look at that 1+G file. ;-)

David
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