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SubjectRe: [PATCH v2 0/8] sched/fair: rework the CFS load balance
Hi Phil,

On Thu, 29 Aug 2019 at 21:23, Phil Auld <pauld@redhat.com> wrote:
>
> On Thu, Aug 01, 2019 at 04:40:16PM +0200 Vincent Guittot wrote:
> > Several wrong task placement have been raised with the current load

> >
> > --
> > 2.7.4
> >
>
> I keep expecting a v3 so I have not dug into all the patches in detail. However, I've

v3 is under preparation

> been working with them from Vincent's tree while they were under development so I thought
> I'd add some results.

Yes. thanks for your help.

>
> The workload is a test our perf team came up with to illustrate the issues we were seeing
> with imbalance in the presence of group scheduling.
>
> On a 4-numa X 20 cpu system (smt on) we run a 76 thread lu.C benchmark from the NAS Parallel
> suite. And at the same time run 2 stress cpu burn processes. The GROUP test puts the
> benchmark and the stress processes each in its own cgroup. The NORMAL case puts them all
> in the first cgroup. The results show first the average number of threads of each type
> running on each of the numa nodes based on sampling taken during the run. This is followed
> by the lu.C benchmark results. There are 3 runs of GROUP and 2 runs of NORMAL shown.
>
> Before (linux-5.3-rc1+ @ a1dc0446d649)
>
> lu.C.x_76_GROUP_1.stress.ps.numa.hist Average 0.00 1.00 1.00
> lu.C.x_76_GROUP_2.stress.ps.numa.hist Average 0.00 1.00 1.00
> lu.C.x_76_GROUP_3.stress.ps.numa.hist Average 0.00 1.00 1.00
> lu.C.x_76_NORMAL_1.stress.ps.numa.hist Average 1.15 0.23 0.00 0.62
> lu.C.x_76_NORMAL_2.stress.ps.numa.hist Average 1.67 0.00 0.00 0.33
>
> lu.C.x_76_GROUP_1.ps.numa.hist Average 30.45 6.95 4.52 34.08
> lu.C.x_76_GROUP_2.ps.numa.hist Average 32.33 8.94 9.21 25.52
> lu.C.x_76_GROUP_3.ps.numa.hist Average 30.45 8.91 12.09 24.55
> lu.C.x_76_NORMAL_1.ps.numa.hist Average 18.54 19.23 19.69 18.54
> lu.C.x_76_NORMAL_2.ps.numa.hist Average 17.25 19.83 20.00 18.92
>
> ============76_GROUP========Mop/s===================================
> min q1 median q3 max
> 2119.92 2418.1 2716.28 3147.82 3579.36
> ============76_GROUP========time====================================
> min q1 median q3 max
> 569.65 660.155 750.66 856.245 961.83
> ============76_NORMAL========Mop/s===================================
> min q1 median q3 max
> 30424.5 31486.4 31486.4 31486.4 32548.4
> ============76_NORMAL========time====================================
> min q1 median q3 max
> 62.65 64.835 64.835 64.835 67.02
>
>
> After (linux-5.3-rc1+ @ a1dc0446d649 + this v2 series pulled from
> Vincent's git on ~8/15)
>
> lu.C.x_76_GROUP_1.stress.ps.numa.hist Average 0.36 1.00 0.64
> lu.C.x_76_GROUP_2.stress.ps.numa.hist Average 1.00 1.00
> lu.C.x_76_GROUP_3.stress.ps.numa.hist Average 1.00 1.00
> lu.C.x_76_NORMAL_1.stress.ps.numa.hist Average 0.23 0.15 0.31 1.31
> lu.C.x_76_NORMAL_2.stress.ps.numa.hist Average 1.00 0.00 0.00 1.00
>
> lu.C.x_76_GROUP_1.ps.numa.hist Average 18.91 18.36 18.91 19.82
> lu.C.x_76_GROUP_2.ps.numa.hist Average 18.36 18.00 19.91 19.73
> lu.C.x_76_GROUP_3.ps.numa.hist Average 18.17 18.42 19.25 20.17
> lu.C.x_76_NORMAL_1.ps.numa.hist Average 19.08 20.00 18.62 18.31
> lu.C.x_76_NORMAL_2.ps.numa.hist Average 18.09 19.91 19.18 18.82
>
> ============76_GROUP========Mop/s===================================
> min q1 median q3 max
> 32304.1 33176 34047.9 34166.8 34285.7
> ============76_GROUP========time====================================
> min q1 median q3 max
> 59.47 59.68 59.89 61.505 63.12
> ============76_NORMAL========Mop/s===================================
> min q1 median q3 max
> 29825.5 32454 32454 32454 35082.5
> ============76_NORMAL========time====================================
> min q1 median q3 max
> 58.12 63.24 63.24 63.24 68.36
>
>
> I had initially tracked this down to two issues. The first was picking the wrong
> group in find_busiest_group due to using the average load. The second was in
> fix_small_imbalance(). The "load" of the lu.C tasks was so low it often failed
> to move anything even when it did find a group that was overloaded (nr_running
> > width). I have two small patches which fix this but since Vincent was embarking
> on a re-work which also addressed this I dropped them.
>
> We've also run a series of performance tests we use to check for regressions and
> did not find any bad results on our workloads and systems.
>
> So...
>
> Tested-by: Phil Auld <pauld@redhat.com>

Thanks for testing

Vincent

>
>
> Cheers,
> Phil
> --

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