Messages in this thread | | | Date | Fri, 20 Nov 2020 15:44:08 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning |
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On Tue, Nov 17, 2020 at 09:35:56PM -0800, Davidlohr Bueso wrote: > On Tue, 17 Nov 2020, Waiman Long wrote: > > > 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. > > I'm not overly worried about the lightly loaded cases here as the users > (mostly thinking mmap_sem) most likely won't care for real workloads, > not, ie: will-it-scale type things. > > So at SUSE we also ran into this very same problem with reader optimistic > spinning and considering the fragmentation went with disabling it, much > like this patch - but without the reader optimistic lock stealing bits > you have. So far nothing has really shown to fall out in our performance > automation. And per your data a single reader spinner does not seem to be > worth the added complexity of keeping reader spinning vs ripping it out.
I'm fine with ripping it... It was finnicky to begin with.
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