Re: Performance degradation with FFSB between 2.6.20 and 2.6.21-rc7

[Andrew Morton <akpm@linux-foundation.org>]

> On Wed, 18 Apr 2007 15:54:00 +0200 Valerie Clement <valerie.clement@bull.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