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Date: Tue, 15 Oct 2013 10:16:44 -0600
From: David Ahern <dsahern@gmail.com>
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To: Ingo Molnar <mingo@kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>,
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        linux-kernel@vger.kernel.org, Frederic Weisbecker <fweisbec@gmail.com>,
        Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
        Mike Galbraith <efault@gmx.de>, Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 3/3] perf record: mmap output file
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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