Re: high latency NFS

["J. Bruce Fields" <bfields@fieldses.org>]

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?

> and apply that criteria to spawning new threads.  i.e.
> we've tried to wake an NFSD, but there are none available so that
> means more NFSDs are needed for the given load. If we've already
> tried to wake one and it hasn't run yet, then we've got enough
> NFSDs....

OK, so you do that instead of trying to directly measure 

> Also, NFSD scheduling needs to be LIFO so that unused NFSDs
> accumulate idle time and so can be culled easily. If you RR the
> nfsds, they'll all appear to be doing useful work so it's hard to
> tell if you've got any idle at all.

Those all sound like good ideas, thanks.

(Still waiting for a volunteer for now, alas.)

--b.