Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

From: Patrick Bellasi
Date: Wed Jun 03 2020 - 04:29:31 EST


Hi Dietmar,
thanks for sharing these numbers.

On Tue, Jun 02, 2020 at 18:46:00 +0200, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> 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.).
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 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.

> 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.

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?
Thus, all the numbers above are granted to be within the specific CI?

> "
> !!! 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?

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

?