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

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Message-ID: <20140703075644.GR4453@dastard>
Date:	Thu, 3 Jul 2014 17:56:44 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Davidlohr Bueso <davidlohr@...com>, linux-kernel@...r.kernel.org,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, jason.low2@...com
Subject: Re: [regression, 3.16-rc] rwsem: optimistic spinning causing
 performance degradation

On Thu, Jul 03, 2014 at 09:38:52AM +0200, Peter Zijlstra wrote:
> On Thu, Jul 03, 2014 at 03:39:11PM +1000, Dave Chinner wrote:
> > There's another regression with the optimisitic spinning in rwsems
> > as well: it increases the size of the struct rw_semaphore by 16
> > bytes. That has increased the size of the struct xfs_inode by 32
> > bytes.
> > 
> > That's pretty damn significant - it's no uncommon to see machines
> > with tens of millions of cached XFS inodes, so increasing the size
> > of the inode by 4% is actually very significant. That's enough to go
> > from having a well balanced workload to not being able to fit the
> > working set of inodes in memory.
> > 
> > Filesystem developers will do almost anything to remove a few bytes
> > from the struct inode because inode cache footprint is extremely
> > important for performance. We also tend to get upset and
> > unreasonable when other people undo that hard work by making changes
> > that bloat the generic structures embedded in the inode
> > structures....
> 
> Jason Low actually did a patch, yesterday, to shrink rwsem back to its
> old size (on 64bit).

That's good to know. Thanks, Peter.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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