[LLVMdev] Use perf tool for more accurate time measuring on Linux

[Yi Kong]

On 16 May 2014 18:40, "Chandler Carruth" <chandlerc at google.com> wrote:
>
> Why not use the cycle count which perf exposes from hardware? That would
seem even better to me, but data would be better. =]

That's an interesting idea. However I'm concerned if that will miss some
aspects of compiler optimization. For example frequent cache misses would
have much smaller impact on the result if the processor goes to lower
frequency during the stall period. Nonetheless it's definitely worth to try
out.

>
>
> On Fri, May 16, 2014 at 11:17 AM, Yi Kong <kongy.dev at gmail.com> wrote:
>>
>> On 16 May 2014 18:08, Hal Finkel <hfinkel at anl.gov> wrote:
>> > ----- Original Message -----
>> >> From: "Yi Kong" <kongy.dev at gmail.com>
>> >> To: "Hal Finkel" <hfinkel at anl.gov>, "Renato Golin" <
renato.golin at linaro.org>, "Tobias Grosser" <tobias at grosser.es>
>> >> Cc: "LLVM Dev" <llvmdev at cs.uiuc.edu>
>> >> Sent: Friday, May 16, 2014 11:37:28 AM
>> >> Subject: Use perf tool for more accurate time measuring on Linux
>> >>
>> >> Hi all,
>> >>
>> >> The LLVM benchmarking system produces very noisy results even on
>> >> quiet
>> >> machines. One of the sources of inaccuracy is the timing tool we are
>> >> using. Because it is a user-space tool, the OS can context switch it
>> >> and we will get an outlier result. Perf stat uses SW_TASK_CLOCK
>> >> counter in kernel to measure time, therefore more accurate. It also
>> >> does not get context switched.
>> >>
>> >> I've implemented a wrapper script over perf stat which mimics the
>> >> behaviour of timeit tool in test suite, so that nothing else needs to
>> >> be modified. The script is not yet feature complete as timeit, but
>> >> enough to run nightly tests.
>> >>
>> >> I carried out experiments on several machines and saw different level
>> >> of improvements. I am no longer seeing outlier results, and MAD is
>> >> considerably lower. The run-by-run changes results over the same
>> >> revision shrank from around 10 to only 2-3. The MAD reduced from
>> >> around 0.01 to 0.003 on a quiet machine.
>> >
>> > That sounds great, thanks for working on this!
>> >
>> > First, we'd definitely need more documentation on what perf is and how
to get it. The testing guide (and lnt dependencies), at least, need to be
updated. FWIW, I don't have any machines on which this is already installed
(and so it certainly is not installed by default). On Ubuntu, it is claimed
that both linux-base and linux-tools-common provide a 'pref' utility, and
on rpm systems it looks like there is a perf package.
>> >
>> > +# FIXME: How to measure sys time?
>> > +echo sys 0.0000 >> $REPORT
>> >
>> > Is this just the difference between the real time and the task time
(or would that be a reasonable approximation)?
>>
>> Not on some occasions. But for most programs, that should a reasonable
>> approximation. Since test suite does not care about the sys time, we
>> can leave it for now.
>> $ time -p sleep 5
>> real 5.00
>> user 0.00
>> sys 0.00
>>
>> >
>> > Thanks again,
>> > Hal
>> >
>> >>
>> >> I've attached the patch and please experiment with it.
>> >>
>> >> Cheers,
>> >> Yi Kong
>> >>
>> >
>> > --
>> > Hal Finkel
>> > Assistant Computational Scientist
>> > Leadership Computing Facility
>> > Argonne National Laboratory
>>
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>
>
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