Messages in this thread | | | Subject | Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation | From | Davidlohr Bueso <> | Date | Fri, 04 Jul 2014 01:40:34 -0700 |
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On Fri, 2014-07-04 at 09:52 +0200, Peter Zijlstra wrote: > On Thu, Jul 03, 2014 at 06:54:50PM -0700, Jason Low wrote: > > Subject: [PATCH] rwsem: In rwsem_can_spin_on_owner(), return false if no owner > > > > It was found that the rwsem optimistic spinning feature can potentially degrade > > performance when there are readers. Perf profiles indicate in some workloads > > that significant time can be spent spinning on !owner. This is because we don't > > set the lock owner when readers(s) obtain the rwsem. > > > > In this patch, we'll modify rwsem_can_spin_on_owner() such that we'll return > > false if there is no lock owner. The rationale is that if we just entered the > > slowpath, yet there is no lock owner, then there is a possibility that a reader > > has the lock. To be conservative, we'll avoid spinning in these situations. > > > > Dave Chinner found performance benefits with this patch in the xfs_repair > > workload, where the total run time went from approximately 4 minutes 24 seconds, > > down to approximately 1 minute 26 seconds with the patch. > > > > Tested-by: Dave Chinner <dchinner@redhat.com> > > Signed-off-by: Jason Low <jason.low2@hp.com> > > Davidlohr, you'll be running this through your AIM and other benchmarks, > I suppose?
I ran it through aim7, and as I suspected we take a performance hit with 'custom' ~-14% throughput for > 300 users (which is still overall quite higher than rwsems without opt spinning, at around ~+45%), and we actually improve a bit (~+15%) in 'disk' with >1000 users -- which afaict resembles Dave's workload a bit. So all in all I'm quite happy with this patch.
Acked-by: Davidlohr Bueso <davidlohr@hp.com>
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