linux-kernel - Re: [PATCH 0/3 v5][RFC] ext3/4: enhance fsync performance when using CFQ

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Date:	Wed, 23 Jun 2010 05:20:28 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	axboe@...nel.dk, vgoyal@...hat.com, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 0/3 v5][RFC] ext3/4: enhance fsync performance when using
 CFQ

On Tue, Jun 22, 2010 at 05:34:59PM -0400, Jeff Moyer wrote:
> Hi,
> 
> Running iozone with the fsync flag, or fs_mark, the performance of CFQ is
> far worse than that of deadline for enterprise class storage when dealing
> with file sizes of 8MB or less.  I used the following command line as a
> representative test case:
> 
>   fs_mark -S 1 -D 10000 -N 100000 -d /mnt/test/fs_mark -s 65536 -t 1 -w 4096 -F
> 
> When run using the deadline I/O scheduler, an average of the first 5 numbers
> will give you 448.4 files / second.  CFQ will yield only 106.7.  With
> this patch series applied (and the two patches I sent yesterday), CFQ now
> achieves 462.5 files / second.
> 
> This patch set is still an RFC.  I'd like to make it perform better when
> there is a competing sequential reader present.  For now, I've addressed
> the concerns voiced about the previous posting.

What happened to the initial idea of just using the BIO_RW_META flag
for log writes?  In the end log writes are the most important writes you
have in a journaled filesystem, and they should not be effect to any
kind of queue idling logic or other interruption.  Log I/O is usually
very little (unless you use old XFS code with a worst-case directory
manipulation workload), and very latency sensitive.
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