Re: [PATCH] perf record: mmap output file - RFC

[Namhyung Kim <namhyung@kernel.org>]

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