Re: [RFC] knfsd: per-client fair scheduling to prevent single-client starvation

On Fri, 05 Jun 2026, Benjamin Coddington wrote:
> On 4 Jun 2026, at 17:16, NeilBrown wrote:
> 
> > On Wed, 03 Jun 2026, Benjamin Coddington wrote:
> >> On 2 Jun 2026, at 23:44, Chuck Lever wrote:
> >>
> >>> On Tue, Jun 2, 2026, at 3:53 PM, NeilBrown wrote:
> >>
> >>>> Idle clients will get pushed back to 1 slot, active client will tend
> >>>> towards a "fair" share based on how comparatively busy they are.
> >>>>
> >>>> This wouldn't help for v3 of course but I don't think we need these
> >>>> advanced features for v3.
> >>>
> >>> Ben’s employer might disagree with that :-)
> >>
> >> Yes - v3 is pretty important to us here.
> >
> > Can you remind me why v3 is important for you?  Is it the lower
> > state-management overhead, or something else?
> 
> Flexfiles uses v3 in its data plane, and..
> 
> > Is there some way would could improve the v4 implementation or protocol
> > to make it comparable to v3 for your use case?
> 
> I don't think so - the stateless nature of v3 gives it distinct advantages
> (and disadvantages) over v4 for some use cases.  That property can't be
> added to v4.

v4 already has the anonymous stateid which seems perfect for a data
plane.
You can read/write with it just like v3 (i.e.  access check on every IO
etc), but there is no state to recover.
You still get client-state and session-state when can be used for flow
control, but these don't need to be recovered so they don't impose the
same costs as open state.

I wonder if flex-files can be updated to allow v4 with anonymous stateid.
We would have to enhance the Linux NFS client to have a "stateless"
mount option, or similar.

> 
> Also, because knfsd doesn't have different resource pools for each version
> we're going to want to continue to balance the pool for all versions
> exported.

But it could if we wanted it to and had a coherent model of how that
would work.  We could parse out the RPC program and version before
queuing for a thread, much like we already parse out the TCP message
size.

We could even do that today: create a separate net-namespace for the
separate pool, and add some iptables rules to redirect select traffic to
that namespace.  iptables cannot switch on RPC version, but it could
switch on source-IP if we knew which clients used v3 and which didn't.

NeilBrown