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

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Message-ID: <20140703073852.GV19379@twins.programming.kicks-ass.net>
Date:	Thu, 3 Jul 2014 09:38:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dave Chinner <david@...morbit.com>
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 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).

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