Re: [PATCH] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE

[Ingo Molnar <mingo@kernel.org>]

* Josef Bacik <jbacik@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