[Centos] IOWAIT High CPU use

Brian Watters brwatters at abs-internet.com
Fri Feb 25 04:01:19 UTC 2005


Ok so I need to find which of these kernels would work in our environment ..

RH v3.0ES 
Dual P3 CPU
SCSI Raid5 PERC 3/Si


-----Original Message-----
From: centos-bounces at caosity.org [mailto:centos-bounces at caosity.org] On
Behalf Of Pasi Pirhonen
Sent: Thursday, February 24, 2005 7:15 PM
To: CentOS discussion and information list
Subject: Re: [Centos] IOWAIT High CPU use


On Thu, Feb 24, 2005 at 06:58:31PM -0800, Brian Watters wrote:
> Hello all,
> Just wanting to check in and see if CentOS v3.4 final has found the 
> fix to the issues that RedHat v3.0 ES has been having with very high 
> CPU use on IOWAIT ??

There is lot of confusion around the whole IOWAIT-thingie. It's perfectly OK
to have machine running 100% (or very close) I/O wait.
These is nothing wrong in that if the disk I/O is still running as expected.

It's just CPU-time that is accounted as process waiting for I/O to finnish
(it's seen as idle time on vanilla 2.4). 

So do you actually have 'sluggish' I/O like speeds dropping cloose to
nothing or just this I/O wait?

> IF so what was the fix and or work around? .. We have lots of RH 3.0 
> servers that we are in the process of converting over to CentOS.

There is no extra patches on CentOS-3 kernels which would make it any better
than the upsource kernel.

For the record, i've been iterating this 'iowait' a lot. My final closure
for it was that ext3 just is broken with 3w-9xxx controller and was
unbareable, so i pathed the XFS in to the kernel tree and has been much
happier since that.

Another thing is that i was easily triggering this 'close to nothing for I/O
speed' with ext3 on vanilla 2.4-seris too. Never with UP-kernel tho.

The kernels has been available like ages on 


if someone (few has) would find those usable. This does account to the
'3ware 9500S' thread too seen here lately - why do you fight with that when
someone has patched it in already and even tested it for you? :P

The problem being tho that it's not available while installing .... It could
be, but all the cloners seem to be so uptight to 'just pure clone' and not
to make any enhancements over upstream source provider.

Anyway. I mostly do this kernel enhancing for parts I do need and share the
results with whom might find those useable.

Pasi Pirhonen - upi at iki.fi - http://iki.fi/upi/
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