From: Vincent Guittot <vincent.guittot@linaro.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Patrick Bellasi <patrick.bellasi@matbug.net>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Qais Yousef <qais.yousef@arm.com>, Ingo Molnar <mingo@redhat.com>,
Randy Dunlap <rdunlap@infradead.org>,
Jonathan Corbet <corbet@lwn.net>,
Juri Lelli <juri.lelli@redhat.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>,
Luis Chamberlain <mcgrof@kernel.org>,
Kees Cook <keescook@chromium.org>,
Iurii Zaikin <yzaikin@google.com>,
Quentin Perret <qperret@google.com>,
Valentin Schneider <valentin.schneider@arm.com>,
Pavan Kondeti <pkondeti@codeaurora.org>,
linux-doc@vger.kernel.org,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-fs <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value
Date: Wed, 3 Jun 2020 16:59:00 +0200 [thread overview]
Message-ID: <CAKfTPtAvMvPk5Ea2kaxXE8GzQ+Nc_PS+EKB1jAa03iJwQORSqA@mail.gmail.com> (raw)
In-Reply-To: <20200603101022.GG3070@suse.de>
On Wed, 3 Jun 2020 at 12:10, Mel Gorman <mgorman@suse.de> wrote:
>
> On Wed, Jun 03, 2020 at 10:29:22AM +0200, Patrick Bellasi wrote:
> >
> > Hi Dietmar,
> > thanks for sharing these numbers.
> >
> > On Tue, Jun 02, 2020 at 18:46:00 +0200, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote...
> >
> > [...]
> >
> > > I ran these tests on 'Ubuntu 18.04 Desktop' on Intel E5-2690 v2
> > > (2 sockets * 10 cores * 2 threads) with powersave governor as:
> > >
> > > $ numactl -N 0 ./run-mmtests.sh XXX
> >
> > Great setup, it's worth to rule out all possible noise source (freq
> > scaling, thermal throttling, NUMA scheduler, etc.).
>
> config-network-netperf-cross-socket will do the binding of the server
> and client to two CPUs that are on one socket. However, it does not take
> care to avoid HT siblings although that could be implemented. The same
> configuration should limit the CPU to C1. It does not change the governor
> but all that would take is adding "cpupower frequency-set -g performance"
> to the end of the configuration.
>
> > Wondering if disabling HT can also help here in reducing results "noise"?
> >
> > > w/ config-network-netperf-unbound.
> > >
> > > Running w/o 'numactl -N 0' gives slightly worse results.
> > >
> > > without-clamp : CONFIG_UCLAMP_TASK is not set
> > > with-clamp : CONFIG_UCLAMP_TASK=y,
> > > CONFIG_UCLAMP_TASK_GROUP is not set
> > > with-clamp-tskgrp : CONFIG_UCLAMP_TASK=y,
> > > CONFIG_UCLAMP_TASK_GROUP=y
> > >
> > >
> > > netperf-udp
> > > ./5.7.0-rc7 ./5.7.0-rc7 ./5.7.0-rc7
> > > without-clamp with-clamp with-clamp-tskgrp
> >
> > Can you please specify how to read the following scores? I give it a run
> > to my local netperf and it reports Throughput, thous I would expect the
> > higher the better... but... this seems something different.
> >
> > > Hmean send-64 153.62 ( 0.00%) 151.80 * -1.19%* 155.60 * 1.28%*
> > > Hmean send-128 306.77 ( 0.00%) 306.27 * -0.16%* 309.39 * 0.85%*
> > > Hmean send-256 608.54 ( 0.00%) 604.28 * -0.70%* 613.42 * 0.80%*
> > > Hmean send-1024 2395.80 ( 0.00%) 2365.67 * -1.26%* 2409.50 * 0.57%*
> > > Hmean send-2048 4608.70 ( 0.00%) 4544.02 * -1.40%* 4665.96 * 1.24%*
> > > Hmean send-3312 7223.97 ( 0.00%) 7158.88 * -0.90%* 7331.23 * 1.48%*
> > > Hmean send-4096 8729.53 ( 0.00%) 8598.78 * -1.50%* 8860.47 * 1.50%*
> > > Hmean send-8192 14961.77 ( 0.00%) 14418.92 * -3.63%* 14908.36 * -0.36%*
> > > Hmean send-16384 25799.50 ( 0.00%) 25025.64 * -3.00%* 25831.20 * 0.12%*
> >
> > If I read it as the lower the score the better, all the above results
> > tell us that with-clamp is even better, while with-clamp-tskgrp
> > is not that much worst.
> >
>
> The figures are throughput to taking the first line
>
> without-clamp 153.62
> with-clamp 151.80 (worse, so the percentage difference is negative)
> with-clamp-tskgrp 155.60 (better so the percentage different is positive)
>
> > The other way around (the higher the score the better) would look odd
> > since we definitively add in more code and complexity when uclamp has
> > the TG support enabled we would not expect better scores.
> >
>
> Netperf for small differences is very fickle as small differences in timing
> or code layout can make a difference. Boot-to-boot variance can also be
> an issue and bisection is generally unreliable. In this case, I relied on
> the perf annotation and differences in ftrace function_graph to determine
> that uclamp was introducing enough overhead to be considered a problem.
When I want to stress the fast path i usually use "perf bench sched pipe -T "
The tip/sched/core on my arm octo core gives the following results for
20 iterations of perf bench sched pipe -T -l 50000
all uclamp config disabled 50035.4(+/- 0.334%)
all uclamp config enabled 48749.8(+/- 0.339%) -2.64%
It's quite easy to reproduce and probably easier to study the impact
>
> > > Hmean recv-64 153.62 ( 0.00%) 151.80 * -1.19%* 155.60 * 1.28%*
> > > Hmean recv-128 306.77 ( 0.00%) 306.27 * -0.16%* 309.39 * 0.85%*
> > > Hmean recv-256 608.54 ( 0.00%) 604.28 * -0.70%* 613.42 * 0.80%*
> > > Hmean recv-1024 2395.80 ( 0.00%) 2365.67 * -1.26%* 2409.50 * 0.57%*
> > > Hmean recv-2048 4608.70 ( 0.00%) 4544.02 * -1.40%* 4665.95 * 1.24%*
> > > Hmean recv-3312 7223.97 ( 0.00%) 7158.88 * -0.90%* 7331.23 * 1.48%*
> > > Hmean recv-4096 8729.53 ( 0.00%) 8598.78 * -1.50%* 8860.47 * 1.50%*
> > > Hmean recv-8192 14961.61 ( 0.00%) 14418.88 * -3.63%* 14908.30 * -0.36%*
> > > Hmean recv-16384 25799.39 ( 0.00%) 25025.49 * -3.00%* 25831.00 * 0.12%*
> > >
> > > netperf-tcp
> > >
> > > Hmean 64 818.65 ( 0.00%) 812.98 * -0.69%* 826.17 * 0.92%*
> > > Hmean 128 1569.55 ( 0.00%) 1555.79 * -0.88%* 1586.94 * 1.11%*
> > > Hmean 256 2952.86 ( 0.00%) 2915.07 * -1.28%* 2968.15 * 0.52%*
> > > Hmean 1024 10425.91 ( 0.00%) 10296.68 * -1.24%* 10418.38 * -0.07%*
> > > Hmean 2048 17454.51 ( 0.00%) 17369.57 * -0.49%* 17419.24 * -0.20%*
> > > Hmean 3312 22509.95 ( 0.00%) 22229.69 * -1.25%* 22373.32 * -0.61%*
> > > Hmean 4096 25033.23 ( 0.00%) 24859.59 * -0.69%* 24912.50 * -0.48%*
> > > Hmean 8192 32080.51 ( 0.00%) 31744.51 * -1.05%* 31800.45 * -0.87%*
> > > Hmean 16384 36531.86 ( 0.00%) 37064.68 * 1.46%* 37397.71 * 2.37%*
> > >
> > > The diffs are smaller than on openSUSE Leap 15.1 and some of the
> > > uclamp taskgroup results are better?
> > >
> > > With this test setup we now can play with the uclamp code in
> > > enqueue_task() and dequeue_task().
> > >
> > > ---
> > >
> > > W/ config-network-netperf-unbound (only netperf-udp and buffer size 64):
> > >
> > > $ perf diff 5.7.0-rc7_without-clamp/perf.data 5.7.0-rc7_with-clamp/perf.data | grep activate_task
> > >
> > > # Event 'cycles:ppp'
> > > #
> > > # Baseline Delta Abs Shared Object Symbol
> > >
> > > 0.02% +0.54% [kernel.vmlinux] [k] activate_task
> > > 0.02% +0.38% [kernel.vmlinux] [k] deactivate_task
> > >
> > > $ perf diff 5.7.0-rc7_without-clamp/perf.data 5.7.0-rc7_with-clamp-tskgrp/perf.data | grep activate_task
> > >
> > > 0.02% +0.35% [kernel.vmlinux] [k] activate_task
> > > 0.02% +0.34% [kernel.vmlinux] [k] deactivate_task
> >
> > These data makes more sense to me, AFAIR we measured <1% impact in the
> > wakeup path using cycletest.
> >
>
> 1% doesn't sound like a lot but UDP_STREAM is an example of a load with
> a *lot* of wakeups so even though the impact on each individual wakeup
> is small, it builds up.
>
> > I would also suggest to always report the overheads for
> > __update_load_avg_cfs_rq()
> > as a reference point. We use that code quite a lot in the wakeup path
> > and it's a good proxy for relative comparisons.
> >
> >
> > > I still see 20 out of 90 tests with the warning message that the
> > > desired confidence was not achieved though.
> >
> > Where the 90 comes from? From the above table we run 9 sizes for
> > {udp-send, udp-recv, tcp} and 3 kernels. Should not give us 81 results?
> >
> > Maybe the Warning are generated only when a test has to be repeated?
>
> The warning is issued when it could not get a reliable result within the
> iterations allowed.
>
> > > "
> > > !!! WARNING
> > > !!! Desired confidence was not achieved within the specified iterations.
> > > !!! This implies that there was variability in the test environment that
> > > !!! must be investigated before going further.
> > > !!! Confidence intervals: Throughput : 6.727% <-- more than 5% !!!
> > > !!! Local CPU util : 0.000%
> > > !!! Remote CPU util : 0.000%
> > > "
> > >
> > > mmtests seems to run netperf with the following '-I' and 'i' parameter
> > > hardcoded: 'netperf -t UDP_STREAM -i 3,3 -I 95,5'
> >
> > This means that we compute a score's (average +-2.5%) with a 95% confidence.
> >
> > Does not that means that every +-2.5% difference in the results
> > above should be considered in the noise?
> >
>
> Usually yes but the impact is small enough to be within noise but
> still detectable. Where we get hurt is when there are multiple problems
> introduced where each contribute overhead that is within the noise but when
> all added together there is a regression outside the noise. Uclamp is not
> special in this respect, it just happens to be the current focus. We met
> this type of problem before with PSI that was resolved by e0c274472d5d
> ("psi: make disabling/enabling easier for vendor kernels").
>
> > I would say that it could be useful to run with more iterations
> > and, given the small numbers we are looking at (apparently we are
> > scared by a 1% overhead), we should better use a more aggressive CI.
> >
> > What about something like:
> >
> > netperf -t UDP_STREAM -i 3,30 -I 99,1
> >
> > ?
> >
>
> You could but the runtime of netperf will be variable, it will not be
> guaranteed to give consistent results and it may mask the true variability
> of the workload. While we could debate which is a valid approach, I
> think it makes sense to minimise the overhead of uclamp when it's not
> configured even if that means putting it behind a static branch that is
> enabled via a command-line parameter or a Kconfig that specifies whether
> it's on or off by default.
>
> --
> Mel Gorman
> SUSE Labs
next prev parent reply other threads:[~2020-06-03 14:59 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-11 15:40 [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Qais Yousef
2020-05-11 15:40 ` [PATCH 2/2] Documentation/sysctl: Document uclamp sysctl knobs Qais Yousef
2020-05-11 17:18 ` [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Qais Yousef
2020-05-12 2:10 ` Pavan Kondeti
2020-05-12 11:46 ` Qais Yousef
2020-05-15 11:08 ` Patrick Bellasi
2020-05-18 8:31 ` Dietmar Eggemann
2020-05-18 16:49 ` Qais Yousef
2020-05-28 13:23 ` Peter Zijlstra
2020-05-28 15:58 ` Qais Yousef
2020-05-28 16:11 ` Peter Zijlstra
2020-05-28 16:51 ` Qais Yousef
2020-05-28 18:29 ` Peter Zijlstra
2020-05-28 19:08 ` Patrick Bellasi
2020-05-28 19:20 ` Dietmar Eggemann
2020-05-29 9:11 ` Qais Yousef
2020-05-29 10:21 ` Mel Gorman
2020-05-29 15:11 ` Qais Yousef
2020-05-29 16:02 ` Mel Gorman
2020-05-29 16:05 ` Qais Yousef
2020-05-29 10:08 ` Mel Gorman
2020-05-29 16:04 ` Qais Yousef
2020-05-29 16:57 ` Mel Gorman
2020-06-02 16:46 ` Dietmar Eggemann
2020-06-03 8:29 ` Patrick Bellasi
2020-06-03 10:10 ` Mel Gorman
2020-06-03 14:59 ` Vincent Guittot [this message]
2020-06-03 16:52 ` Qais Yousef
2020-06-04 12:14 ` Vincent Guittot
2020-06-05 10:45 ` Qais Yousef
2020-06-09 15:29 ` Vincent Guittot
2020-06-08 12:31 ` Qais Yousef
2020-06-08 13:06 ` Valentin Schneider
2020-06-08 14:44 ` Steven Rostedt
2020-06-11 10:13 ` Qais Yousef
2020-06-09 17:10 ` Vincent Guittot
2020-06-11 10:24 ` Qais Yousef
2020-06-11 12:01 ` Vincent Guittot
2020-06-23 15:44 ` Qais Yousef
2020-06-24 8:45 ` Vincent Guittot
2020-06-05 7:55 ` Patrick Bellasi
2020-06-05 11:32 ` Qais Yousef
2020-06-05 13:27 ` Patrick Bellasi
2020-06-03 9:40 ` Mel Gorman
2020-06-03 12:41 ` Qais Yousef
2020-06-04 13:40 ` Mel Gorman
2020-06-05 10:58 ` Qais Yousef
2020-06-11 10:58 ` Qais Yousef
2020-06-16 11:08 ` Qais Yousef
2020-06-16 13:56 ` Lukasz Luba
-- strict thread matches above, loose matches on Subject: below --
2020-04-03 12:30 Qais Yousef
2020-04-14 18:21 ` Patrick Bellasi
2020-04-15 7:46 ` Patrick Bellasi
2020-04-20 15:04 ` Qais Yousef
2020-04-20 8:24 ` Dietmar Eggemann
2020-04-20 15:19 ` Qais Yousef
2020-04-21 0:52 ` Steven Rostedt
2020-04-21 11:16 ` Dietmar Eggemann
2020-04-21 11:23 ` Qais Yousef
2020-04-20 14:50 ` Qais Yousef
2020-04-15 10:11 ` Quentin Perret
2020-04-20 15:08 ` Qais Yousef
2020-04-20 8:29 ` Dietmar Eggemann
2020-04-20 15:13 ` Qais Yousef
2020-04-21 11:18 ` Dietmar Eggemann
2020-04-21 11:27 ` Qais Yousef
2020-04-22 10:59 ` Dietmar Eggemann
2020-04-22 13:13 ` Qais Yousef
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