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SubjectRe: [PATCH] perf record: mmap output file - RFC
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Hi Jiri and David,

On Thu, 26 Sep 2013 19:51:05 +0200, Jiri Olsa wrote:
> On Sun, Sep 22, 2013 at 08:05:59PM -0600, 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 mmap's the file in chunks of 64M
>> at a time and copies events from the event buffers to the file
>> avoiding write system calls.
>
> moved processing into userspace:
>
> 17.24% -17.10% libpthread-2.15.so [.] __write_nocancel
> ...
> 0.07% +0.64% libc-2.15.so [.] __memcpy_sse2
> 0.02% +51.84% libc-2.15.so [.] __memcpy_ssse3_back
> 0.01% +0.34% libc-2.15.so [.] __mempcpy_sse2
> ...
>>
>> 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 ]
>
> ^^^^^^^^
> but it's still faster, since we finally get perf a chance to sleep ;-)
>
> new time:
> real 0m30.392s
> user 0m0.041s
> sys 0m0.389s
>
> old time:
> real 0m32.235s
> user 0m3.080s
> sys 0m14.444s

But why the new user time took so short? I guess it should take at
least 10 seconds or so. Any ideas?

>
>
>> [ perf record: Captured and wrote 8.203 MB /tmp/perf.data (~358388 samples) ]
>>
>> Before I get too far down this path I wanted to get comments on the approach.
>
> I think it's worthwhile doing this

Indeed! It looks like a nice improvement.

Thanks,
Namhyung


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