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Message-Id: <20070418135709.b499e050.akpm@linux-foundation.org>
Date: Wed, 18 Apr 2007 13:57:09 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Valerie Clement <valerie.clement@...l.net>
Cc: linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Performance degradation with FFSB between 2.6.20 and 2.6.21-rc7
> On Wed, 18 Apr 2007 15:54:00 +0200 Valerie Clement <valerie.clement@...l.net> wrote:
>
> Running benchmark tests (FFSB) on an ext4 filesystem, I noticed a
> performance degradation (about 15-20 percent) in sequential write tests
> between 2.6.19-rc6 and 2.6.21-rc4 kernels.
>
> I ran the same tests on ext3 and XFS filesystems and I saw the same
> performance difference between the two kernel versions for these two
> filesystems.
>
> I have also reproduced it between 2.6.20.7 and 2.6.21-rc7.
> The FFSB tests run 16 threads, each creating 1GB files. The tests were
> done on the same x86_64 system, with the same kernel configuration and
> on the same scsi device. Below are the throughput values given by FFSB.
>
> kernel XFS ext3
> ----------
> 2.6.20.7 48 MB/sec 44 MB/sec
>
> 2.6.21-rc7 38 MB/sec 37 MB/sec
>
> Did anyone else run across the problem?
> Is there a known issue?
>
That's a new discovery, thanks.
It could be due to I/O scheduler changes. Which one are you using? CFQ?
Or it could be that there has been some changed behaviour at the VFS/pagecache
layer: the VFS might be submitting little hunks of lots of files, rather than
large hunks of few files.
Or it could be a block-layer thing: perhaps some driver change has caused
us to be placing less data into the queue. Which device driver is that machine
using?
Being a simple soul, the first thing I'll try when I get near a test box
will be
for i in $(seq 1 16)
do
time dd if=/dev/zero of=$i bs=1M count=1024 &
done
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