Re: High load average on disk I/O on 2.6.17-rc3

[Mike Galbraith <efault@gmx.de>]

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