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 11:02:54AM +0000, Dieter wrote:
> In message <20061122165238.GA37819 at xor.obsecurity.org>, Kris Kennaway writes:
> 
> > > > I'm surprised that you're seeing that much of a "hang".  Even if the di=
> > sks
> > > > are busy, the system should slow down all disk processes equally, so no
> > > > one process "blocks", but they're all a little slower.
> > >=20
> > > I collected a bit of data:
> > >=20
> > > While copying a large file from disk1 to disk2,
> > >=20
> > > time ls on a small directory on disk3 (not cached in memory)
> > >=20
> > > real    0m0.032s
> > > user    0m0.000s
> > > sys     0m0.003s
> > >=20
> > > time ls on a small directory on disk2
> > >=20
> > > real    4m51.911s
> > > user    0m0.000s
> > > sys     0m0.002s
> > >=20
> > > 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.
> > >=20
> > > Sometimes I see long delays when accessing disk3, but it is
> > > behaving at the moment.
> > 
> > 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.
> 
> In telnet window 1:
> 
> cd /disk1/
> cp -ip very_big_file /disk2/bar/	(the workload)
> 
> In telnet window 2:
> 
> 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)
> 
> The /disk2/ directory is small, only contains 3 directories and .snap
> 
> Would the cp into /disk2/bar/ lock the /disk2/ directory?

It shouldn't do.

What scheduler are you using?

Kris
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