Messages in this thread | | | Date | Fri, 4 Jul 2014 16:13:06 +1000 | From | Dave Chinner <> | Subject | Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation |
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On Thu, Jul 03, 2014 at 06:54:50PM -0700, Jason Low wrote: > On Thu, 2014-07-03 at 18:46 -0700, Jason Low wrote: > > On Fri, 2014-07-04 at 11:01 +1000, Dave Chinner wrote: > > > > FWIW, the rwsems in the struct xfs_inode are often heavily > > > read/write contended, so there are lots of IO related workloads that > > > are going to regress on XFS without this optimisation... > > > > > > Anyway, consider the patch: > > > > > > Tested-by: Dave Chinner <dchinner@redhat.com> > > > > Hi Dave, > > > > Thanks for testing. I'll update the patch with an actual changelog. > > --- > 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.
I don't think you're being a little shifty with the truth here. There's no "potentially degrade performance" here - I reported a massive real world performance regression caused by optimistic spinning. That is:
"Commit 4fc828e ("locking/rwsem: Support optimistic spinning") introduced a major performance regression for workloads such as xfs_repair which mix read and write locking of the mmap_sem across many threads. The result was xfs_repair ran 5x slower on 3.16-rc2 than on 3.15 and using 20x more system CPU time."
"Perf profiles indicate....
> 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.
Which brought it back to close to the same performance as on 3.15. This is not a new performance improvement patch - it's a regression fix and the commit message needs to reflect that.
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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