linux-kernel - Re: [PATCH 3/3] perf record: mmap output file

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Message-ID: <525D6A6C.3090208@gmail.com>
Date:	Tue, 15 Oct 2013 10:16:44 -0600
From:	David Ahern <dsahern@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/3] perf record: mmap output file

On 10/15/13 10:06 AM, Ingo Molnar wrote:
> splice() is very fast and should be able to process a lot of pages in one
> go, so the feedback loop should be pretty weak. mmap() triggers kernel
> code as well, every time we run out of the 64 MB window we got to remap
> it, right?


Yes, 1 mmap, 1 munmap for every 64MB. Compare to the write() case which 
calls write() for each mmap each time through the mmap_read loop.

I am conjecturing that splice would follow the write model in the sense 
of a ring buffer has N bytes, call splice to copy the data from the ring 
buffer to the file. So, splice saves on the memcpy, but not the syscalls.

David
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