Re: High load average on disk I/O on 2.6.17-rc3
From: Mike Galbraith
Date: Mon May 08 2006 - 13:17:28 EST
On Mon, 2006-05-08 at 17:42 +0200, Erik Mouw wrote:
> On Mon, May 08, 2006 at 05:31:29PM +0200, Arjan van de Ven wrote:
> > On Mon, 2006-05-08 at 17:22 +0200, Erik Mouw wrote:
> > > ... except that any kernel < 2.6 didn't account tasks waiting for disk
> > > IO.
> >
> > they did. It was "D" state, which counted into load average.
>
> They did not or at least to a much lesser extent. That's the reason why
> ZenIV.linux.org.uk had a mail DoS during the last FC release and why we
> see load average questions on lkml.
I distinctly recall it counting, but since I don't have a 2.4 tree
handy, I'll refrain from saying "did _too_" ;-)
> I've seen it on our servers as well: when using 2.4 and doing 50 MB/s
> to disk (through NFS), the load just was slightly above 0. When we
> switched the servers to 2.6 it went to ~16 for the same disk usage.
The main difference I see is...
8129 root 15 0 3500 512 432 D 56.0 0.0 0:33.72 bonnie
1393 root 10 -5 0 0 0 D 0.4 0.0 0:00.26 kjournald
8135 root 15 0 0 0 0 D 0.0 0.0 0:00.01 pdflush
573 root 15 0 0 0 0 D 0.0 0.0 0:00.00 pdflush
574 root 15 0 0 0 0 D 0.0 0.0 0:00.04 pdflush
8131 root 15 0 0 0 0 D 0.0 0.0 0:00.01 pdflush
8141 root 15 0 0 0 0 D 0.0 0.0 0:00.00 pdflush
With 2.4, there was only one flush thread. Same load, different
loadavg... in this particular case of one user task running. IIRC, if
you had a bunch of things running and running you low on memory, you
could end up with a slew of 'D' state tasks in 2.4 as well, because
allocating tasks had to help free memory by flushing buffers and pushing
swap. Six to one, half a dozen to the other.
-Mike
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