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

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


Thanks,
Parth

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