Messages in this thread | | | Date | Tue, 6 Aug 2019 14:44:17 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH-tip] locking/rwsem: Make handoff writer optimistically spin on owner |
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On Tue, Jun 25, 2019 at 10:39:13AM -0400, Waiman Long wrote: > When the handoff bit is set by a writer, no other tasks other than > the setting writer itself is allowed to acquire the lock. If the > to-be-handoff'ed writer goes to sleep, there will be a wakeup latency > period where the lock is free, but no one can acquire it. That is less > than ideal. > > To reduce that latency, the handoff writer will now optimistically spin > on the owner if it happens to be a on-cpu writer. It will spin until > it releases the lock and the to-be-handoff'ed writer can then acquire > the lock immediately without any delay. Of course, if the owner is not > a on-cpu writer, the to-be-handoff'ed writer will have to sleep anyway. > > The optimistic spinning code is also modified to not stop spinning > when the handoff bit is set. This will prevent an occasional setting of > handoff bit from causing a bunch of optimistic spinners from entering > into the wait queue causing significant reduction in throughput. > > On a 1-socket 22-core 44-thread Skylake system, the AIM7 shared_memory > workload was run with 7000 users. The throughput (jobs/min) of the > following kernels were as follows: > > 1) 5.2-rc6 > - 8,092,486 > 2) 5.2-rc6 + tip's rwsem patches > - 7,567,568 > 3) 5.2-rc6 + tip's rwsem patches + this patch > - 7,954,545 > > Using perf-record(1), the %cpu time used by rwsem_down_write_slowpath(), > rwsem_down_write_failed() and their callees for the 3 kernels were 1.70%, > 5.46% and 2.08% respectively. > > Signed-off-by: Waiman Long <longman@redhat.com>
Thanks.. sorry for taking so long.
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