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

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Message-ID: <20131112145707.GV5056@laptop.programming.kicks-ass.net>
Date:	Tue, 12 Nov 2013 15:57:07 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Ahern <dsahern@...il.com>
Cc:	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	mingo@...nel.org, jolsa@...hat.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 4/5] perf record: mmap output file - v5

On Tue, Nov 12, 2013 at 07:46:56AM -0700, David Ahern wrote:
> When recording raw_syscalls for the entire system, e.g.,
>     perf record -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
> 
> you end up with a negative feedback loop as perf itself calls write() fairly
> often. This patch handles the problem by mmap'ing the file in chunks of 64M at
> a time and copies events from the event buffers to the file avoiding write
> system calls.

You know this completely fails the moment you trace faults, because
every new access to one of those pages (to mark it dirty) will trigger a
fault. And we'll take a bunch more faults -- one for each page -- than
we ever did write() syscalls.

Anyway the over all performance improvements still make it worth it. But
the above seems like a false argument in favour of this.
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