bad NFS/UDP performance

[Robert Watson]

On Mon, 29 Sep 2008, Oliver Fromme wrote:

> Danny Braniss wrote:
> > Grr, there goes binary search theory out of the window,
> > So far I have managed to pinpoint the day that the changes affect the
> > throughput:
> >         18/08/08 00:00:00       19/08/08 00:00:00
> > (I assume cvs's date is GMT).
> > now would be a good time for some help, specially how to undo changes, my
> > knowledge of csup/cvs are close to zero.
>
> So you've nailed to down to this 24-hour window:
>
> http://www.secnetix.de/olli/FreeBSD/svnews/?day=2008-08-18&p=/stable/7
>
> I'm afraid that r181822 by rwatson is the most likely candidate that might 
> be causing the regression.

If we can confirm that it was that specific change, then I can create a patch 
to restore exclusive locking for UDP and we can see if it was the general move 
to rwlocking, or specifically the read-locking of certain data structures. 
Perhaps what we've done is moved contention from a mutex to a sleep lock, 
reducing the efficiency of handling contention?  Adding Kris to the CC line 
because he often has useful insights on this sort of thing.

Robert N M Watson
Computer Laboratory
University of Cambridge