linux-kernel - Re: [regression, 3.16-rc] rwsem: optimistic spinning causing performance degradation

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Date:	Fri, 04 Jul 2014 01:40:34 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jason Low <jason.low2@...com>, Dave Chinner <david@...morbit.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [regression, 3.16-rc] rwsem: optimistic spinning causing
 performance degradation

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@...hat.com>
> > Signed-off-by: Jason Low <jason.low2@...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@...com>

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