[llvm-dev] Noisy benchmark results?

Bruce Hoult via llvm-dev
2017-02-27T10:32:59Z

Two other things:

1) I get massively more stable execution times on 16.04 than on 14.04 on
both x86 and ARM because 16.04 does far fewer gratuitous moves from one
core to another, even without explicit pinning.

2) turn off ASLR: "echo 0 > /proc/sys/kernel/randomize_va_space".
As well
as getting stable addresses for debugging repeatability, it also stabilizes
execution time variability due to "random" conflicts in caches, hash
collisions in branch prediction or BTB, maybe even uop cache.


On Mon, Feb 27, 2017 at 12:36 PM, Kristof Beyls via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Mikael,
>
> Some noisiness in benchmark results is expected, but the numbers you see
> seem to be higher than I'd expect.
> A number of tricks people use to get lower noise results are (with the lnt
> runtest nt command line options to enable it between brackets):
> * Only build the benchmarks in parallel, but do the actual running of the
> benchmark code at most one at a time. (--threads 1 --build-threads 6).
> * Make lnt use linux perf to get more accurate timing for short-running
> benchmarks (--use-perf=1)
> * Pin the running benchmark to a specific core, so the OS doesn't move
the
> benchmark process from core to core. (--make-param=RUNUNDER=taskset -c 1)
> * Only run the programs that are marked as a benchmark; some of the tests
> in the test-suite are not intended to be used as a benchmark
> (--benchmarking-only)
> * Make sure each program gets run multiple times, so that LNT has a higher
> chance of recognizing which programs are inherently noisy (--multisample=3)
>
> I hope this is the kind of answer you were looking for?
> Do the above measures reduce the noisiness to acceptable levels for your
> setup?
>
> Thanks,
>
> Kristof
>
>
> > On 27 Feb 2017, at 09:46, Mikael Holmén via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > I'm trying to run the benchmark suite:
> > http://llvm.org/docs/TestingGuide.html#test-suite-quickstart
> >
> > I'm doing it the lnt way, as described at:
> > http://llvm.org/docs/lnt/quickstart.html
> >
> > I don't know what to expect but the results seems to be quite
noisy and
> unstable. E.g I've done two runs on two different commits that only
differ
> by a space in CODE_OWNERS.txt on my 12 core ubuntu 14.04 machine with:
> >
> > lnt runtest nt --sandbox SANDBOX --cc <path-to-my-clang>
--test-suite
> /data/repo/test-suite -j 8
> >
> > And then I get the following top execution time regressions:
> > http://i.imgur.com/sv1xzlK.png
> >
> > The numbers bounce around a lot if I do more runs.
> >
> > Given the amount of noise I see here I don't know to sort out
> significant regressions if I actually do a real change in the compiler.
> >
> > Are the above results expected?
> >
> > How to use this?
> >
> >
> > As a bonus question, if I instead run the benchmarks with an added
-m32:
> > lnt runtest nt --sandbox SANDBOX --cflag=-m32 --cc
<path-to-my-clang>
> --test-suite /data/repo/test-suite -j 8
> >
> > I get three failures:
> >
> > --- Tested: 2465 tests --
> > FAIL: MultiSource/Applications/ClamAV/clamscan.compile_time (1 of
2465)
> > FAIL: MultiSource/Applications/ClamAV/clamscan.execution_time (494 of
> 2465)
> > FAIL:
MultiSource/Benchmarks/DOE-ProxyApps-C/XSBench/XSBench.execution_time
> (495 of 2465)
> >
> > Is this known/expected or do I do something stupid?
> >
> > Thanks,
> > Mikael
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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