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SubjectRe: [PATCH v3 0/8] sched/fair: rework the CFS load balance
Hi Parth,

On Wed, 16 Oct 2019 at 09:21, Parth Shah <parth@linux.ibm.com> wrote:
>
>
>
> On 9/19/19 1:03 PM, Vincent Guittot wrote:
> > Several wrong task placement have been raised with the current load
> > balance algorithm but their fixes are not always straight forward and
> > end up with using biased values to force migrations. A cleanup and rework
> > of the load balance will help to handle such UCs and enable to fine grain
> > the behavior of the scheduler for other cases.
> >
> > Patch 1 has already been sent separately and only consolidate asym policy
> > in one place and help the review of the changes in load_balance.
> >
> > Patch 2 renames the sum of h_nr_running in stats.
> >
> > Patch 3 removes meaningless imbalance computation to make review of
> > patch 4 easier.
> >
> > Patch 4 reworks load_balance algorithm and fixes some wrong task placement
> > but try to stay conservative.
> >
> > Patch 5 add the sum of nr_running to monitor non cfs tasks and take that
> > into account when pulling tasks.
> >
> > Patch 6 replaces runnable_load by load now that the signal is only used
> > when overloaded.
> >
> > Patch 7 improves the spread of tasks at the 1st scheduling level.
> >
> > Patch 8 uses utilization instead of load in all steps of misfit task
> > path.
> >
> > Patch 9 replaces runnable_load_avg by load_avg in the wake up path.
> >
> > Patch 10 optimizes find_idlest_group() that was using both runnable_load
> > and load. This has not been squashed with previous patch to ease the
> > review.
> >
> > Some benchmarks results based on 8 iterations of each tests:
> > - small arm64 dual quad cores system
> >
> > tip/sched/core w/ this patchset improvement
> > schedpipe 54981 +/-0.36% 55459 +/-0.31% (+0.97%)
> >
> > hackbench
> > 1 groups 0.906 +/-2.34% 0.906 +/-2.88% (+0.06%)
> >
> > - large arm64 2 nodes / 224 cores system
> >
> > tip/sched/core w/ this patchset improvement
> > schedpipe 125323 +/-0.98% 125624 +/-0.71% (+0.24%)
> >
> > hackbench -l (256000/#grp) -g #grp
> > 1 groups 15.360 +/-1.76% 14.206 +/-1.40% (+8.69%)
> > 4 groups 5.822 +/-1.02% 5.508 +/-6.45% (+5.38%)
> > 16 groups 3.103 +/-0.80% 3.244 +/-0.77% (-4.52%)
> > 32 groups 2.892 +/-1.23% 2.850 +/-1.81% (+1.47%)
> > 64 groups 2.825 +/-1.51% 2.725 +/-1.51% (+3.54%)
> > 128 groups 3.149 +/-8.46% 3.053 +/-13.15% (+3.06%)
> > 256 groups 3.511 +/-8.49% 3.019 +/-1.71% (+14.03%)
> >
> > dbench
> > 1 groups 329.677 +/-0.46% 329.771 +/-0.11% (+0.03%)
> > 4 groups 931.499 +/-0.79% 947.118 +/-0.94% (+1.68%)
> > 16 groups 1924.210 +/-0.89% 1947.849 +/-0.76% (+1.23%)
> > 32 groups 2350.646 +/-5.75% 2351.549 +/-6.33% (+0.04%)
> > 64 groups 2201.524 +/-3.35% 2192.749 +/-5.84% (-0.40%)
> > 128 groups 2206.858 +/-2.50% 2376.265 +/-7.44% (+7.68%)
> > 256 groups 1263.520 +/-3.34% 1633.143 +/-13.02% (+29.25%)
> >
> > tip/sched/core sha1:
> > 0413d7f33e60 ('sched/uclamp: Always use 'enum uclamp_id' for clamp_id values')
> > [...]
>
> I am quietly impressed with this patch series as it makes easy to
> understand the behavior of the load balancer just by looking at the code.

Thanks

>
> I have tested v3 on IBM POWER9 system with following configuration:
> - CPU(s): 176
> - Thread(s) per core: 4
> - Core(s) per socket: 22
> - Socket(s): 2
> - Model name: POWER9, altivec supported
> - NUMA node0 CPU(s): 0-87
> - NUMA node8 CPU(s): 88-175
>
> I see results in par with the baseline (tip/sched/core) with most of my
> testings.
>
> hackbench
> =========
> hackbench -l (256000/#grp) -g #grp (lower is better):
> +--------+--------------------+-------------------+------------------+
> | groups | w/ patches | Baseline | Performance gain |
> +--------+--------------------+-------------------+------------------+
> | 1 | 14.948 (+/- 0.10) | 15.13 (+/- 0.47 ) | +1.20 |
> | 4 | 5.938 (+/- 0.034) | 6.085 (+/- 0.07) | +2.4 |
> | 8 | 6.594 (+/- 0.072) | 6.223 (+/- 0.03) | -5.9 |
> | 16 | 5.916 (+/- 0.05) | 5.559 (+/- 0.00) | -6.4 |
> | 32 | 5.288 (+/- 0.034) | 5.23 (+/- 0.01) | -1.1 |
> | 64 | 5.147 (+/- 0.036) | 5.193 (+/- 0.09) | +0.8 |
> | 128 | 5.368 (+/- 0.0245) | 5.446 (+/- 0.04) | +1.4 |
> | 256 | 5.637 (+/- 0.088) | 5.596 (+/- 0.07) | -0.7 |
> | 512 | 5.78 (+/- 0.0637) | 5.934 (+/- 0.06) | +2.5 |
> +--------+--------------------+-------------------+------------------+
>
>
> dbench
> ========
> dbench <grp> (Throughput: Higher is better):
> +---------+---------------------+-----------------------+----------+
> | groups | w/ patches | baseline | gain |
> +---------+---------------------+-----------------------+----------+
> | 1 | 12.6419(+/-0.58) | 12.6511 (+/-0.277) | -0.00 |
> | 4 | 23.7712(+/-2.22) | 21.8526 (+/-0.844) | +8.7 |
> | 8 | 40.1333(+/-0.85) | 37.0623 (+/-3.283) | +8.2 |
> | 16 | 60.5529(+/-2.35) | 60.0972 (+/-9.655) | +0.7 |
> | 32 | 98.2194(+/-1.69) | 87.6701 (+/-10.72) | +12.0 |
> | 64 | 150.733(+/-9.91) | 109.782 (+/-0.503) | +37.3 |
> | 128 | 173.443(+/-22.4) | 130.006 (+/-21.84) | +33.4 |
> | 256 | 121.011(+/-15.2) | 120.603 (+/-11.82) | +0.3 |
> | 512 | 10.9889(+/-0.39) | 12.5518 (+/-1.030) | -12 |
> +---------+---------------------+-----------------------+----------+
>
>
>
> I am happy with the results as it turns to be beneficial in most cases.
> Still I will be doing testing for different scenarios and workloads.

Thanks for the tests results

> BTW do you have any specific test case which might show different behavior
> for SMT-4/8 systems with these patch set?

No, I don't have any specific tests case in mind but it should better
spread tasks in cores for middle loaded UCs with tasks with different
weight either beacuse different nice prio or because of cgroup. The
load_balance doesn't use load in this case unlike current
implementation.

Thanks
Vincent

>
>
> Thanks,
> Parth
>

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