* Josef Bacik <jba...@fb.com> wrote:
> On 05/26/2015 05:31 PM, Josef Bacik wrote:
> >At Facebook we have a pretty heavily multi-threaded application that is
> >sensitive to latency. We have been pulling forward the old SD_WAKE_IDLE code
> >because it gives us a pretty significant performance gain (like 20%). It
> >turns
> >out this is because there are cases where the scheduler puts our task on a
> >busy
> >CPU when there are idle CPU's in the system. We verify this by reading the
> >cpu_delay_req_avg_us from the scheduler netlink stuff. With our crappy
> >patch we
> >get much lower numbers vs baseline.
> >
> >SD_BALANCE_WAKE is supposed to find us an idle cpu to run on, however it is
> >just
> >looking for an idle sibling, preferring affinity over all else. This is not
> >helpful in all cases, and SD_BALANCE_WAKE's job is to find us an idle cpu,
> >not
> >garuntee affinity. Fix this by first trying to find an idle sibling, and
> >then
> >if the cpu is not idle fall through to the logic to find an idle cpu. With
> >this
> >patch we get slightly better performance than with our forward port of
> >SD_WAKE_IDLE. Thanks,
> >
>
> I rigged up a test script to run the perf bench sched tests and give me the
> numbers. Here are the numbers
>
> 4.0
>
> Messaging: 56.934 Total runtime in seconds
> Pipe: 105620.762 ops/sec
>
> 4.0 + my patch
>
> Messaging: 47.374
> Pipe: 113691.199
Btw., with perf bench you don't really need much extra scripting, something
like
this should give you pretty good numbers plus an stddev estimate:
perf stat --null --repeat 10 perf bench sched messaging -l 10000
on my box this gives:
4.391469643 seconds time elapsed
( +- 2.81% )
you can adjust the -l value to move the runtime up/down to a value that you
think
runs long enough to give stable results.
Thanks,
Ingo
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