Date: Thu, 3 Jul 2014 09:38:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Dave Chinner <david@fromorbit.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>, linux-kernel@vger.kernel.org,
        Tim Chen <tim.c.chen@linux.intel.com>, Ingo Molnar <mingo@kernel.org>,
        jason.low2@hp.com
Subject: Re: [regression, 3.16-rc] rwsem: optimistic spinning causing
 performance degradation
Message-ID: <20140703073852.GV19379@twins.programming.kicks-ass.net>
References: <20140703023204.GN4453@dastard>
 <1404358268.23839.13.camel@buesod1.americas.hpqcorp.net>
 <20140703045933.GP4453@dastard>
 <20140703053911.GQ4453@dastard>
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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.
>=20
> 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.
>=20
> 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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