linux-kernel - Re: [PATCH 2/2] perf record: mmap output file

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Date:	Mon, 11 Nov 2013 12:29:06 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	acme@...stprotocols.net, linux-kernel@...r.kernel.org,
	jolsa@...hat.com, Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 2/2] perf record: mmap output file - v4


* David Ahern <dsahern@...il.com> 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.
> 
> Before (with write syscall):
> 
>     perf record -o /tmp/perf.data -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
>     [ perf record: Woken up 0 times to write data ]
>     [ perf record: Captured and wrote 81.843 MB /tmp/perf.data (~3575786 samples) ]
> 
> After (using mmap):
> 
>     perf record -o /tmp/perf.data -e raw_syscalls:*,sched:sched_switch -a -- sleep 1
>     [ perf record: Woken up 31 times to write data ]
>     [ perf record: Captured and wrote 8.203 MB /tmp/perf.data (~358388 samples) ]
> 
> In addition to perf-trace benefits using mmap lowers the overhead of
> perf-record. For example,
> 
>   perf stat -i -- perf record -g -o /tmp/perf.data openssl speed aes
> 
> shows a drop in time, CPU cycles, and instructions all drop by more than a
> factor of 3. Jiri also ran a test that showed a big improvement.
> 
> v4: Refactoring per Ingo's comments
> 
> v3: Removed use of bytes_at_mmap_start at the stat() that set it
>     Added user option to control the size of the mmap for writing file.
> 
> v2: Removed msync call before munmap per Jiri's suggestion
> 
> Signed-off-by: David Ahern <dsahern@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Jiri Olsa <jolsa@...hat.com>
> Cc: Namhyung Kim <namhyung@...nel.org>
> Cc: Mike Galbraith <efault@....de>
> Cc: Stephane Eranian <eranian@...gle.com>
> ---
>  tools/perf/Documentation/perf-record.txt |   5 +
>  tools/perf/builtin-record.c              | 155 +++++++++++++++++++++++++++++++
>  2 files changed, 160 insertions(+)

Looks very clean now!

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo
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