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SubjectRe: [PATCH v2 5/5] locking/rwsem: Remove reader optimistic spinning
On Fri, 20 Nov 2020, Waiman Long wrote:

>Reader optimistic spinning is helpful when the reader critical section
>is short and there aren't that many readers around. It also improves
>the chance that a reader can get the lock as writer optimistic spinning
>disproportionally favors writers much more than readers.
>
>Since commit d3681e269fff ("locking/rwsem: Wake up almost all readers
>in wait queue"), all the waiting readers are woken up so that they can
>all get the read lock and run in parallel. When the number of contending
>readers is large, allowing reader optimistic spinning will likely cause
>reader fragmentation where multiple smaller groups of readers can get
>the read lock in a sequential manner separated by writers. That reduces
>reader parallelism.
>
>One possible way to address that drawback is to limit the number of
>readers (preferably one) that can do optimistic spinning. These readers
>act as representatives of all the waiting readers in the wait queue as
>they will wake up all those waiting readers once they get the lock.
>
>Alternatively, as reader optimistic lock stealing has already enhanced
>fairness to readers, it may be easier to just remove reader optimistic
>spinning and simplifying the optimistic spinning code as a result.
>
>Performance measurements (locking throughput kops/s) using a locking
>microbenchmark with 50/50 reader/writer distribution and turbo-boost
>disabled was done on a 2-socket Cascade Lake system (48-core 96-thread)
>to see the impacts of these changes:
>
> 1) Vanilla - 5.10-rc3 kernel
> 2) Before - 5.10-rc3 kernel with previous patches in this series
> 2) limit-rspin - 5.10-rc3 kernel with limited reader spinning patch
> 3) no-rspin - 5.10-rc3 kernel with reader spinning disabled
>
> # of threads CS Load Vanilla Before limit-rspin no-rspin
> ------------ ------- ------- ------ ----------- --------
> 2 1 5,185 5,662 5,214 5,077
> 4 1 5,107 4,983 5,188 4,760
> 8 1 4,782 4,564 4,720 4,628
> 16 1 4,680 4,053 4,567 3,402
> 32 1 4,299 1,115 1,118 1,098
> 64 1 3,218 983 1,001 957
> 96 1 1,938 944 957 930
>
> 2 20 2,008 2,128 2,264 1,665
> 4 20 1,390 1,033 1,046 1,101
> 8 20 1,472 1,155 1,098 1,213
> 16 20 1,332 1,077 1,089 1,122
> 32 20 967 914 917 980
> 64 20 787 874 891 858
> 96 20 730 836 847 844
>
> 2 100 372 356 360 355
> 4 100 492 425 434 392
> 8 100 533 537 529 538
> 16 100 548 572 568 598
> 32 100 499 520 527 537
> 64 100 466 517 526 512
> 96 100 406 497 506 509
>
>The column "CS Load" represents the number of pause instructions issued
>in the locking critical section. A CS load of 1 is extremely short and
>is not likey in real situations. A load of 20 (moderate) and 100 (long)
>are more realistic.
>
>It can be seen that the previous patches in this series have reduced
>performance in general except in highly contended cases with moderate
>or long critical sections that performance improves a bit. This change
>is mostly caused by the "Prevent potential lock starvation" patch that
>reduce reader optimistic spinning and hence reduce reader fragmentation.
>
>The patch that further limit reader optimistic spinning doesn't seem to
>have too much impact on overall performance as shown in the benchmark
>data.
>
>The patch that disables reader optimistic spinning shows reduced
>performance at lightly loaded cases, but comparable or slightly better
>performance on with heavier contention.
>
>This patch just removes reader optimistic spinning for now. As readers
>are not going to do optimistic spinning anymore, we don't need to
>consider if the OSQ is empty or not when doing lock stealing.
>
>Signed-off-by: Waiman Long <longman@redhat.com>

Reviewed-by: Davidlohr Bueso <dbueso@suse.de>

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