linux-kernel - Re: [MMTests] IO metadata on XFS

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Date:	Mon, 2 Jul 2012 09:54:58 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Mel Gorman <mgorman@...e.de>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, xfs@....sgi.com
Subject: Re: [MMTests] IO metadata on XFS

On Fri, Jun 29, 2012 at 12:25:06PM +0100, Mel Gorman wrote:
> Configuration:	global-dhp__io-metadata-xfs
> Benchmarks:	dbench3, fsmark-single, fsmark-threaded
> 
> Summary
> =======
> Most of the figures look good and in general there has been consistent good
> performance from XFS. However, fsmark-single is showing a severe performance
> dip in a few cases somewhere between 3.1 and 3.4. fs-mark running a single
> thread took a particularly bad dive in 3.4 for two machines that is worth
> examining closer.

That will be caused by the fact we changed all the metadata updates
to be logged, which means a transaction every time .dirty_inode is
called.

This should mostly go away when XFS is converted to use .update_time
rather than .dirty_inode to only issue transactions when the VFS
updates the atime rather than every .dirty_inode call...

> Unfortunately it is harder to easy conclusions as the
> gains/losses are not consistent between machines which may be related to
> the available number of CPU threads.

It increases the CPU overhead (dirty_inode can be called up to 4
times per write(2) call, IIRC), so with limited numbers of
threads/limited CPU power it will result in lower performance. Where
you have lots of CPU power, there will be little difference in
performance...

Cheers,

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