Re: [sched/fair] 0b0695f2b3: phoronix-test-suite.compress-gzip.0.seconds 19.8% regression

[Oliver Sang]

On Wed, May 20, 2020 at 03:04:48PM +0200, Vincent Guittot wrote:
> On Thu, 14 May 2020 at 19:09, Vincent Guittot
> <vincent.guittot@xxxxxxxxxx> wrote:
> >
> > Hi Oliver,
> >
> > On Thu, 14 May 2020 at 16:05, kernel test robot <oliver.sang@xxxxxxxxx> wrote:
> > >
> > > Hi Vincent Guittot,
> > >
> > > Below report FYI.
> > > Last year, we actually reported an improvement "[sched/fair] 0b0695f2b3:
> > > vm-scalability.median 3.1% improvement" on link [1].
> > > but now we found the regression on pts.compress-gzip.
> > > This seems align with what showed in "[v4,00/10] sched/fair: rework the CFS
> > > load balance" (link [2]), where showed the reworked load balance could have
> > > both positive and negative effect for different test suites.
> >
> > We have tried to run  all possible use cases but it's impossible to
> > covers all so there were a possibility that one that is not covered,
> > would regressed.
> >
> > > And also from link [3], the patch set risks regressions.
> > >
> > > We also confirmed this regression on another platform
> > > (Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz with 8G memory),
> > > below is the data (lower is better).
> > > v5.4    4.1
> > > fcf0553db6f4c79387864f6e4ab4a891601f395e    4.01
> > > 0b0695f2b34a4afa3f6e9aa1ff0e5336d8dad912    4.89
> > > v5.5    5.18
> > > v5.6    4.62
> > > v5.7-rc2    4.53
> > > v5.7-rc3    4.59
> > >
> > > It seems there are some recovery on latest kernels, but not fully back.
> > > We were just wondering whether you could share some lights the further works
> > > on the load balance after patch set [2] which could cause the performance
> > > change?
> > > And whether you have plan to refine the load balance algorithm further?
> >
> > I'm going to have a look at your regression to understand what is
> > going wrong and how it can be fixed
> 
> I have run the benchmark on my local setups to try to reproduce the
> regression and I don't see the regression. But my setups are different
> from your so it might be a problem specific to yours

Hi Vincent, which OS are you using? We found the regression on Clear OS,
but it cannot reproduce on Debian.
On https://www.phoronix.com/scan.php?page=article&item=mac-win-linux2018&num=5
it was mentioned that -
Gzip compression is much faster out-of-the-box on Clear Linux due to it exploiting
multi-threading capabilities compared to the other operating systems Gzip support. 

> 
> After analysing the benchmark, it doesn't overload the system and is
> mainly based on 1 main gzip thread with few others waking up and
> sleeping around.
> 
> I thought that scheduler could be too aggressive when trying to
> balance the threads on your system, which could generate more task
> migrations and impact the performance. But this doesn't seem to be the
> case because perf-stat.i.cpu-migrations is -8%. On the other side, the
> context switch is +16% and more interestingly idle state C1E and C6
> usages increase more than 50%. I don't know if we can rely or this
> value or not but I wonder if it could be that threads are now spread
> on different CPUs which generates idle time on the busy CPUs but the
> added time to enter/leave these states hurts the performance.
> 
> Could you make some traces of both kernels ? Tracing sched events
> should be enough to understand the behavior
> 
> Regards,
> Vincent
> 
> >
> > Thanks
> > Vincent
> >
> > > thanks
> > >
> > > [1] https://lists.01.org/hyperkitty/list/lkp@xxxxxxxxxxxx/thread/SANC7QLYZKUNMM6O7UNR3OAQAKS5BESE/
> > > [2] https://lore.kernel.org/patchwork/cover/1141687/
> > > [3] https://www.phoronix.com/scan.php?page=news_item&px=Linux-5.5-Scheduler