On Tue, Jun 02, 2020 at 01:23:19PM +0800, Oliver Sang wrote: > On Fri, May 29, 2020 at 07:26:01PM +0200, Vincent Guittot wrote: > > On Mon, 25 May 2020 at 10:02, Vincent Guittot > > <[email protected]> wrote: > > > > > > On Thu, 21 May 2020 at 10:28, Oliver Sang <[email protected]> wrote: > > > > > > > > On Wed, May 20, 2020 at 03:04:48PM +0200, Vincent Guittot wrote: > > > > > On Thu, 14 May 2020 at 19:09, Vincent Guittot > > > > > <[email protected]> wrote: > > > > > > > > > > > > Hi Oliver, > > > > > > > > > > > > On Thu, 14 May 2020 at 16:05, kernel test robot > > > > > > <[email protected]> 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. > > > > > > I'm using Debian, I haven't noticed it was only on Clear OS. > > > I'm going to look at it. Could you send me traces in the meantime ? > > > > I run more tests to understand the problem. Even if Clear Linux uses > > multithreading, the system is not overloaded and there is a > > significant amount of idle time. This means that we use the has_spare > > capacity path that spreads tasks on the system. At least that is what > > I have seen in the KVM image. Beside this, I think that I have been > > able to reproduce the problem on my platform with debian using pigz > > instead of gzip for the compress-gzip-1.2.0 test. On my platform, I > > can see a difference when I enable all CPU idle states whereas there > > is no performance difference when only the shallowest idle state is > > enabled. > > > > The new load balance rework is more efficient at spreading tasks on > > the system and one side effect could be that there is more idle time > > between tasks wake up on each CPU. As a result, CPUs have to wake up > > from a deeper idle state. This could explain the +54% increase of C6 > > usage that is reported. Is it possible to get All C-state statistics > > ? > > Hi Vincent, sorry for late. I selected the turbostat while running > compress-gzip on Clear OS, as attached. Besides v5.4 and v5.7-rc7, > I got the data for v5.5 and v5.7, too. Not sure it's enough since you > said "All C-state statistics". Hope it could be useful and if you want > more data, please let us know. > > Again, I collect these data on our "Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz > with 8G memory" platform. for each release, I ran test twice to assure > the results consistent. > > phoronix-test-suite.compress-gzip.0.seconds > release run1 run2 > v5.4 4.37 4.37 > v5.5 5.37 5.46 > v5.7-rc7 4.82 4.85 > v5.7 4.86 4.83 > > > > > Could you run the test after disabling deep idle states like C6 and > > above and check what is the difference between v5.7-rc7 and v5.4 ? > > thanks for suggestion, will collect this data later.
Hi Vincent, I test by disabling deep idle states, but the regression
still exists, just the gap becomes smaller -
release run1 run2
v5.4 4.32 4.3 <-- little change comparing to above
v5.5 5.04 5.06 <-- improves
v5.7-rc7 4.79 4.78
v5.7 4.77 4.77
I also attached turbostat data as attached.
>
> > comparing fcf0553db6 ("sched/fair: Remove meaningless imbalance
> > calculation") and
> > 0b0695f2b3 ("sched/fair: Rework load_balance()") is not really
> > useful because they are part of the same rework and should be
> > considered a one single change.
> >
> > >
> > > >
> > > > >
> > > > > 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/[email protected]/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
compress-gzip-turbostat-no-c6.tgz
Description: application/gtar-compressed
