Re: high latency NFS

[Bernd Schubert]

On Monday 04 August 2008 03:11:58 J. Bruce Fields wrote:
> On Mon, Aug 04, 2008 at 10:32:06AM +1000, Dave Chinner wrote:
> > On Fri, Aug 01, 2008 at 03:15:59PM -0400, J. Bruce Fields wrote:
> > > On Fri, Aug 01, 2008 at 05:23:20PM +1000, Dave Chinner wrote:
> > > > On Thu, Jul 31, 2008 at 05:03:05PM +1000, Neil Brown wrote:
> > > > > You might want to track the max length of the request queue too and
> > > > > start more threads if the queue is long, to allow a quick ramp-up.
> > > >
> > > > Right, but even request queue depth is not a good indicator. You
> > > > need to leep track of how many NFSDs are actually doing useful
> > > > work. That is, if you've got an NFSD on the CPU that is hitting
> > > > the cache and not blocking, you don't need more NFSDs to handle
> > > > that load because they can't do any more work than the NFSD
> > > > that is currently running is.
> > > >
> > > > i.e. take the solution that Greg banks used for the CPU scheduler
> > > > overload issue (limiting the number of nfsds woken but not yet on
> > > > the CPU),
> > >
> > > I don't remember that, or wasn't watching when it happened.... Do you
> > > have a pointer?
> >
> > Ah, I thought that had been sent to mainline because it was
> > mentioned in his LCA talk at the start of the year. Slides
> > 65-67 here:
> >
> > http://mirror.linux.org.au/pub/linux.conf.au/2007/video/talks/41.pdf
>
> OK, so to summarize: when the rate of incoming rpc's is very high (and,
> I guess, when we're serving everything out of cache and don't have IO
> wait), all the nfsd threads will stay runable all the time.  That keeps
> userspace processes from running (possibly for "minutes").  And that's a
> problem even on a server dedicated only to nfs, since it affects portmap
> and rpc.mountd.

Even worse, it affects user space HA software such as heartbeat and everyone 
with reasonable timeouts will see spurious 'failures'. 


-- 
Bernd Schubert
Q-Leap Networks GmbH
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