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SubjectRe: [RFC][PATCH 1/5] sched/fair: Fix select_idle_cpu()s cost accounting
On Fri, Jan 08, 2021 at 02:41:19PM +0100, Vincent Guittot wrote:
> > 1. avg_scan_cost is now based on the average scan cost of a rq but
> > avg_idle is still scaled to the domain size. This is a bit problematic
> > because it's comparing scan cost of a single rq with the estimated
> > average idle time of a domain. As a result, the scan depth can be much
> > larger than it was before the patch and led to some regressions.
>
> Point 1 makes sense to me too
>
> >
> > 2. Accounting for the scan cost of success makes sense but there is a
> > big difference between a scan that finds an idle CPU and one that fails.
> > For failures, the scan cost is wasted CPU time where as a success
> > means that an uncontested CPU is used. This can cause a search to be
> > truncated earlier than it should be when the domain is lightly loaded.
>
> But I'm not sure to catch your problem with point 2.
> track the average cost to scan one rq so looping all rqs are only few
> should not impact (much) the avg_scan_cost
>
> Trying to bias the avg_scan_cost with: loops <<= 2;
> will just make avg_scan_cost lost any kind of meaning because it
> doesn't reflect the avg cost of scanning a rq anymore
>

Before the series, the avg_scan_cost also did not represent the cost of
scanning a RQ before either. Treating scan failures and successes equally
can problems with the depth of the scan conducted. Previously the "cost"
of a successful scan was 0 so successful scans allowed deeper scans in
the near future. This partially illustrates the problem.

5.11.0-rc2 5.11.0-rc2 5.11.0-rc2
baseline-v2r1 acctscan-v2r1 altscan-v2r8
Hmean 1 429.47 ( 0.00%) 420.90 * -2.00%* 414.27 * -3.54%*
Hmean 2 709.39 ( 0.00%) 796.05 * 12.22%* 791.98 * 11.64%*
Hmean 4 1449.19 ( 0.00%) 1445.14 ( -0.28%) 1319.09 * -8.98%*
Hmean 8 2765.65 ( 0.00%) 2750.07 * -0.56%* 2756.17 * -0.34%*
Hmean 16 5158.47 ( 0.00%) 5056.59 * -1.97%* 5030.67 * -2.48%*
Hmean 32 8969.96 ( 0.00%) 8796.96 * -1.93%* 8768.34 * -2.25%*
Hmean 64 11210.05 ( 0.00%) 9910.39 * -11.59%* 11073.42 * -1.22%*
Hmean 128 17978.21 ( 0.00%) 17031.41 * -5.27%* 17037.76 * -5.23%*
Hmean 256 16143.32 ( 0.00%) 15636.59 * -3.14%* 15761.12 * -2.37%*
Hmean 320 16388.59 ( 0.00%) 15591.78 * -4.86%* 15588.85 * -4.88%*

Note the impact of Peters patch (accescan-v2r1) for 64 threads. The
machine is 2-socket (40 cores, 80 threads) so 64 is the load is
balancing between two domains (load balancing vs wakeup migrations).
altscan is my suggested patch on top and with Peter's patch, there is a
11.59% regression that is negligible with my patch on top.

The impact is machine-specific or specific to the CPU generation. Here
is just comparing just the suggested alteration on a slightly older
generation.

5.11.0-rc2 5.11.0-rc2
acctscan-v2r1 altscan-v2r8
Hmean 1 155.44 ( 0.00%) 183.32 * 17.94%*
Hmean 2 445.46 ( 0.00%) 548.51 * 23.13%*
Hmean 4 1080.25 ( 0.00%) 1112.49 * 2.98%*
Hmean 8 2253.48 ( 0.00%) 2457.46 * 9.05%*
Hmean 16 3996.73 ( 0.00%) 4244.59 * 6.20%*
Hmean 32 5318.93 ( 0.00%) 5798.17 * 9.01%*
Hmean 64 9301.55 ( 0.00%) 9563.24 * 2.81%*
Hmean 128 8560.89 ( 0.00%) 8873.72 * 3.65%*
Hmean 192 8526.92 ( 0.00%) 8843.43 * 3.71%*

And another 2-socket machine on a newer generation.

Hmean 1 551.16 ( 0.00%) 503.75 * -8.60%*
Hmean 2 1074.19 ( 0.00%) 1078.08 * 0.36%*
Hmean 4 2024.72 ( 0.00%) 2049.29 * 1.21%*
Hmean 8 3762.49 ( 0.00%) 4002.24 * 6.37%*
Hmean 16 6589.98 ( 0.00%) 6688.21 * 1.49%*
Hmean 32 10080.23 ( 0.00%) 10270.34 * 1.89%*
Hmean 64 11349.16 ( 0.00%) 12452.68 * 9.72%*
Hmean 128 21670.93 ( 0.00%) 21823.70 * 0.70%*
Hmean 256 20605.62 ( 0.00%) 20615.01 * 0.05%*
Hmean 320 20974.29 ( 0.00%) 20565.11 * -1.95%*

For hackbench with processes communicating via pipes on the first
machine

5.11.0-rc2 5.11.0-rc2
acctscan-v2r1 altscan-v2r8
Amean 1 0.3927 ( 0.00%) 0.3943 ( -0.42%)
Amean 4 0.9247 ( 0.00%) 0.9267 ( -0.22%)
Amean 7 1.4587 ( 0.00%) 1.5147 * -3.84%*
Amean 12 2.3637 ( 0.00%) 2.4507 * -3.68%*
Amean 21 4.0700 ( 0.00%) 4.1757 * -2.60%*
Amean 30 5.6573 ( 0.00%) 5.7390 * -1.44%*
Amean 48 8.9037 ( 0.00%) 8.8053 * 1.10%*
Amean 79 14.9190 ( 0.00%) 14.4360 * 3.24%*
Amean 110 22.5703 ( 0.00%) 21.9210 ( 2.88%)
Amean 141 29.2400 ( 0.00%) 28.0110 * 4.20%*
Amean 172 36.3720 ( 0.00%) 34.7963 ( 4.33%)
Amean 203 43.5783 ( 0.00%) 42.5537 * 2.35%*
Amean 234 50.3653 ( 0.00%) 47.3463 * 5.99%*
Amean 265 57.6153 ( 0.00%) 55.6247 * 3.46%*
Amean 296 62.7370 ( 0.00%) 62.0720 ( 1.06%)

Adjusting the scan cost for successes is neither a universal win or
failure but it's closer to historical behaviour and the strict
accounting does hit corner cases. If a deep scan is finding an idle CPU,
it makes some sense to continue scanning deeply by adjusting the weight
instead of prematurely failing.

The testing of the full series previously showed that some loads never
recovered from side-effects of the first patch and the last patch in the
series introduced new problems of its own. Hence, I would like to limit
the negative impact of the first patch and, if necessary, cut the last
patch altogether.

--
Mel Gorman
SUSE Labs

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