processes not getting fair share of available disk I/O (was: Re: TCP parameters and interpreting tcpdump output )

[Kris Kennaway]

On Wed, Nov 22, 2006 at 06:12:06PM +0000, Dieter wrote:

> > > > > time ls on a small directory on disk2
> > > > >=3D20
> > > > > real    4m51.911s
> > > > > user    0m0.000s
> > > > > sys     0m0.002s
> > > > >=3D20
> > > > > I expect access to a busy disk to take longer, but 5 minutes is
> > > > > a bit much.  And that's the root directory of the filesystem,
> > > > > it didn't have to follow a long chain of directories to get there.
> > > > >=3D20
> > > > > Sometimes I see long delays when accessing disk3, but it is
> > > > > behaving at the moment.
> > > >=20
> > > > ls still has to acquire a number of locks in order to be sure that the
> > > > contents of the directory aren't changing.  If there are lots of other
> > > > processes all competing for these locks, it will be slow.  It looks
> > > > like that's the case on your system, although details of your workload
> > > > have been trimmed from your email.
> > >=20
> > > In telnet window 1:
> > >=20
> > > cd /disk1/
> > > cp -ip very_big_file /disk2/bar/	(the workload)
> > >=20
> > > In telnet window 2:
> > >=20
> > > time ls /disk3/foo1/  (make sure time and ls are cached in memory)
> > > time ls /disk3/foo2/  (see timing numbers above)
> > > time ls /disk2/       (see timing numbers above)
> > >=20
> > > The /disk2/ directory is small, only contains 3 directories and .snap
> > >=20
> > > Would the cp into /disk2/bar/ lock the /disk2/ directory?
> > 
> > It shouldn't do.
> > 
> > What scheduler are you using?
> 
> kern.sched.name: 4BSD
> kern.sched.quantum: 100000
> kern.sched.preemption: 1

OK, that's correct.  Can you also provide details of your disk
hardware (e.g. dmesg) and kernel configuration?

Kris
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