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SubjectRe: [PATCH] sched/fair: use dst group while checking imbalance for NUMA balancer
Resending - previously, I forgot to send the message in the plain text
mode. I'm sorry.

Hi Mel,

Thanks a lot for looking into this!

Our results are mostly in line with what you see. We observe big gains
(20-50%) when the system is loaded to 1/3 of the maximum capacity and
mixed results at the full load - some workloads benefit from the patch
at the full load, others not, but performance changes at the full load
are mostly within the noise of results (+/-5%). Overall, we think this
patch is helpful.

Jirka


On Mon, Sep 21, 2020 at 1:02 PM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Thu, Sep 10, 2020 at 11:50:04PM +0200, Jirka Hladky wrote:
> > 1) Threads stability has improved a lot. We see much fewer threads
> > migrations. Check for example this heatmap based on the mpstat results
> > collected while running sp.C test from the NAS benchmark. sp.C was run
> > with 16 threads on a dual-socket AMD 7351 server with 8 NUMA nodes:
> > 5.9 vanilla https://drive.google.com/file/d/10rojhTSQUSu-1aGQi-srr99SmVQ3Llgo/view?usp=sharing
> > 5.9 with the patch
> > https://drive.google.com/file/d/1eZdTBWvWMNvdvXqitwAlcKZ7gb-ySQUl/view?usp=sharing
> >
> > The heatmaps are generated from the mpstat output (there are 5
> > different runs at one picture). We collect CPU utilization every 5
> > seconds. Lighter color corresponds to lower CPU utilization. Light
> > color means that the thread may have run on different CPUs during that
> > 5 seconds. Solid straight lines, on the other hand, mean that thread
> > was running on the same CPU all the time. The difference is striking.
> >
> > We see significantly fewer threads migrations across many different
> > tests - NAS, SPECjbb2005, SPECjvm2008
> >
>
> For all three, I see the point where it's better or worse depends on
> overall activity. I looked at heatmaps for a variety of workloads and
> visually the bigger differences tend to be with utilisation is relatively
> low (e.g. one third of CPUs active).
>
> > 2) We see also performance improvement in terms of runtime, especially
> > at low load scenarios (number of threads being roughly equal to the 2*
> > number of NUMA nodes). It fixes the performance drop which we see
> > since 5.7 kernel, discussed for example here:
> > https://lore.kernel.org/lkml/CAE4VaGB7+sR1nf3Ux8W=hgN46gNXRYr0uBWJU0oYnk7h00Y_dw@mail.gmail.com/
> >
>
> This would make some sense given the original intent about allowing
> imbalances that was later revised significantly. It depends on when
> memory throughput is more important so the impact varies with the level
> of untilisation. For example, on a 2-socket machine running specjvm 2005
> I see
>
> specjbb
> 5.9.0-rc4 5.9.0-rc4
> vanilla dstbalance-v1r1
> Hmean tput-1 46425.00 ( 0.00%) 43394.00 * -6.53%*
> Hmean tput-2 98416.00 ( 0.00%) 96031.00 * -2.42%*
> Hmean tput-3 150184.00 ( 0.00%) 148783.00 * -0.93%*
> Hmean tput-4 200683.00 ( 0.00%) 197906.00 * -1.38%*
> Hmean tput-5 236305.00 ( 0.00%) 245549.00 * 3.91%*
> Hmean tput-6 281559.00 ( 0.00%) 285692.00 * 1.47%*
> Hmean tput-7 338558.00 ( 0.00%) 334467.00 * -1.21%*
> Hmean tput-8 340745.00 ( 0.00%) 372501.00 * 9.32%*
> Hmean tput-9 424343.00 ( 0.00%) 413006.00 * -2.67%*
> Hmean tput-10 421854.00 ( 0.00%) 434261.00 * 2.94%*
> Hmean tput-11 493256.00 ( 0.00%) 485330.00 * -1.61%*
> Hmean tput-12 549573.00 ( 0.00%) 529959.00 * -3.57%*
> Hmean tput-13 593183.00 ( 0.00%) 555010.00 * -6.44%*
> Hmean tput-14 588252.00 ( 0.00%) 599166.00 * 1.86%*
> Hmean tput-15 623065.00 ( 0.00%) 642713.00 * 3.15%*
> Hmean tput-16 703924.00 ( 0.00%) 660758.00 * -6.13%*
> Hmean tput-17 666023.00 ( 0.00%) 697675.00 * 4.75%*
> Hmean tput-18 761502.00 ( 0.00%) 758360.00 * -0.41%*
> Hmean tput-19 796088.00 ( 0.00%) 798368.00 * 0.29%*
> Hmean tput-20 733564.00 ( 0.00%) 823086.00 * 12.20%*
> Hmean tput-21 840980.00 ( 0.00%) 856711.00 * 1.87%*
> Hmean tput-22 804285.00 ( 0.00%) 872238.00 * 8.45%*
> Hmean tput-23 795208.00 ( 0.00%) 889374.00 * 11.84%*
> Hmean tput-24 848619.00 ( 0.00%) 966783.00 * 13.92%*
> Hmean tput-25 750848.00 ( 0.00%) 903790.00 * 20.37%*
> Hmean tput-26 780523.00 ( 0.00%) 962254.00 * 23.28%*
> Hmean tput-27 1042245.00 ( 0.00%) 991544.00 * -4.86%*
> Hmean tput-28 1090580.00 ( 0.00%) 1035926.00 * -5.01%*
> Hmean tput-29 999483.00 ( 0.00%) 1082948.00 * 8.35%*
> Hmean tput-30 1098663.00 ( 0.00%) 1113427.00 * 1.34%*
> Hmean tput-31 1125671.00 ( 0.00%) 1134175.00 * 0.76%*
> Hmean tput-32 968167.00 ( 0.00%) 1250286.00 * 29.14%*
> Hmean tput-33 1077676.00 ( 0.00%) 1060893.00 * -1.56%*
> Hmean tput-34 1090538.00 ( 0.00%) 1090933.00 * 0.04%*
> Hmean tput-35 967058.00 ( 0.00%) 1107421.00 * 14.51%*
> Hmean tput-36 1051745.00 ( 0.00%) 1210663.00 * 15.11%*
> Hmean tput-37 1019465.00 ( 0.00%) 1351446.00 * 32.56%*
> Hmean tput-38 1083102.00 ( 0.00%) 1064541.00 * -1.71%*
> Hmean tput-39 1232990.00 ( 0.00%) 1303623.00 * 5.73%*
> Hmean tput-40 1175542.00 ( 0.00%) 1340943.00 * 14.07%*
> Hmean tput-41 1127826.00 ( 0.00%) 1339492.00 * 18.77%*
> Hmean tput-42 1198313.00 ( 0.00%) 1411023.00 * 17.75%*
> Hmean tput-43 1163733.00 ( 0.00%) 1228253.00 * 5.54%*
> Hmean tput-44 1305562.00 ( 0.00%) 1357886.00 * 4.01%*
> Hmean tput-45 1326752.00 ( 0.00%) 1406061.00 * 5.98%*
> Hmean tput-46 1339424.00 ( 0.00%) 1418451.00 * 5.90%*
> Hmean tput-47 1415057.00 ( 0.00%) 1381570.00 * -2.37%*
> Hmean tput-48 1392003.00 ( 0.00%) 1421167.00 * 2.10%*
> Hmean tput-49 1408374.00 ( 0.00%) 1418659.00 * 0.73%*
> Hmean tput-50 1359822.00 ( 0.00%) 1391070.00 * 2.30%*
> Hmean tput-51 1414246.00 ( 0.00%) 1392679.00 * -1.52%*
> Hmean tput-52 1432352.00 ( 0.00%) 1354020.00 * -5.47%*
> Hmean tput-53 1387563.00 ( 0.00%) 1409563.00 * 1.59%*
> Hmean tput-54 1406420.00 ( 0.00%) 1388711.00 * -1.26%*
> Hmean tput-55 1438804.00 ( 0.00%) 1387472.00 * -3.57%*
> Hmean tput-56 1399465.00 ( 0.00%) 1400296.00 * 0.06%*
> Hmean tput-57 1428132.00 ( 0.00%) 1396399.00 * -2.22%*
> Hmean tput-58 1432385.00 ( 0.00%) 1386253.00 * -3.22%*
> Hmean tput-59 1421612.00 ( 0.00%) 1371416.00 * -3.53%*
> Hmean tput-60 1429423.00 ( 0.00%) 1389412.00 * -2.80%*
> Hmean tput-61 1396230.00 ( 0.00%) 1351122.00 * -3.23%*
> Hmean tput-62 1418396.00 ( 0.00%) 1383098.00 * -2.49%*
> Hmean tput-63 1409918.00 ( 0.00%) 1374662.00 * -2.50%*
> Hmean tput-64 1410236.00 ( 0.00%) 1376216.00 * -2.41%*
> Hmean tput-65 1396405.00 ( 0.00%) 1364418.00 * -2.29%*
> Hmean tput-66 1395975.00 ( 0.00%) 1357326.00 * -2.77%*
> Hmean tput-67 1392986.00 ( 0.00%) 1349642.00 * -3.11%*
> Hmean tput-68 1386541.00 ( 0.00%) 1343261.00 * -3.12%*
> Hmean tput-69 1374407.00 ( 0.00%) 1342588.00 * -2.32%*
> Hmean tput-70 1377513.00 ( 0.00%) 1334654.00 * -3.11%*
> Hmean tput-71 1369319.00 ( 0.00%) 1334952.00 * -2.51%*
> Hmean tput-72 1354635.00 ( 0.00%) 1329005.00 * -1.89%*
> Hmean tput-73 1350933.00 ( 0.00%) 1318942.00 * -2.37%*
> Hmean tput-74 1351714.00 ( 0.00%) 1316347.00 * -2.62%*
> Hmean tput-75 1352198.00 ( 0.00%) 1309974.00 * -3.12%*
> Hmean tput-76 1349490.00 ( 0.00%) 1286064.00 * -4.70%*
> Hmean tput-77 1336131.00 ( 0.00%) 1303684.00 * -2.43%*
> Hmean tput-78 1308896.00 ( 0.00%) 1271024.00 * -2.89%*
> Hmean tput-79 1326703.00 ( 0.00%) 1290862.00 * -2.70%*
> Hmean tput-80 1336199.00 ( 0.00%) 1291629.00 * -3.34%*
>
> Note as the utilisation reaches roughly half the CPUs that performance
> is much better but then degrades for higher thread counts. Part of this
> is that NUMA balancing activity is higher with the patch once roughly a
> quarter of the CPUs are in use.
>
> NAS is all over the map depending on the compute kernel and degree of
> parallelisation. Sometimes it is much faster depending on the machine
> and the degree of parallelisation, other times slower. Generally the
> level of NUMA balancing activity is variable -- sometimes better,
> sometimes worse and locality is generally lower (which is not a solid
> conclusion as it's usually correlated with differences in hinting
> faults).
>
> There are counter-examples. On EPYC 2 running hackbench, locality and
> scan activity is much worse with the patch. On hackbench using processes
> and sockets, it is about 8% slower for small numbers of groups and 28%
> slower for larger numbers of groups. On the same workload, NUMA activity
> is much higher, locality goes from 50% to 1.5%. Bizarrely, with pipes
> the patch is much better except in the opposite direction.
>
> For something like dbench, it depends on the machine. All but one
> machine showed an improvement. On EPYC 2, it's much worse.
>
> > The biggest improvements are for the NAS and the SPECjvm2008
> > benchmarks (typically between 20-50%). SPECjbb2005 shows also
> > improvements, around 10%. This is again on NUMA servers at the low
> > utilization. You can find snapshots of some results here:
> > https://drive.google.com/drive/folders/1k3Gb-vlI7UjPZZcLkoL2W2VB_zqxIJ3_?usp=sharing
> >
> > @Mel - could you please test the proposed patch? I know you have good
> > coverage for the inter-process communication benchmarks which may show
> > different behavior than benchmarks which we use. I hope that fewer
> > threads migrations might show positive effects also for these tests.
> > Please give it a try.
> >
>
> Basically I see mixed bag depending on the workload, machine and overall
> levels of utilisation. Sometimes it's better (sometimes much better),
> other times it is worse (sometimes much worse). Given that there isn't a
> universally good decision in this section and more people seem to prefer
> the patch then it may be best to keep the LB decisions consistent and
> revisit imbalance handling when the load balancer code changes settle
> down so
>
> Acked-by: Mel Gorman <mgorman@techsingularity.net>
>
> --
> Mel Gorman
> SUSE Labs
>


--
-Jirka

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