Re: Re: unsharing tcp connections from different NFS mounts

["bfields@fieldses.org" <bfields@fieldses.org>]

Thanks very much to all of you for the explanations and concrete
suggestions for things to look at, I feel much less stuck!

--b.

On Tue, May 04, 2021 at 02:27:04PM +0000, Trond Myklebust wrote:
> On Tue, 2021-05-04 at 12:08 +1000, NeilBrown wrote:
> > On Tue, 04 May 2021, bfields@fieldses.org wrote:
> > > On Wed, Jan 20, 2021 at 10:07:37AM -0500, bfields@fieldses.org wrote:
> > > > 
> > > > So mainly:
> > > > 
> > > > > > > Why is there a performance regression being seen by these
> > > > > > > setups
> > > > > > > when they share the same connection? Is it really the
> > > > > > > connection,
> > > > > > > or is it the fact that they all share the same fixed-slot
> > > > > > > session?
> > > > 
> > > > I don't know.  Any pointers how we might go about finding the
> > > > answer?
> > > 
> > > I set this aside and then get bugged about it again.
> > > 
> > > I apologize, I don't understand what you're asking for here, but it
> > > seemed obvious to you and Tom, so I'm sure the problem is me.  Are
> > > you
> > > free for a call sometime maybe?  Or do you have any suggestions for
> > > how
> > > you'd go about investigating this?
> > 
> > I think a useful first step would be to understand what is getting in
> > the way of the small requests.
> >  - are they in the client waiting for slots which are all consumed by
> >    large writes?
> >  - are they in TCP stream behind megabytes of writes that need to be
> >    consumed before they can even be seen by the server?
> >  - are they in a socket buffer on the server waiting to be served
> >    while all the nfsd thread are busy handling writes?
> > 
> > I cannot see an easy way to measure which it is.
> 
> The nfs4_sequence_done tracepoint will give you a running count of the
> highest slot id in use.
> 
> The mountstats 'execute time' will give you the time between the
> request being created and the time a reply was received. That time
> includes the time spent waiting for a NFSv4 session slot.
> 
> The mountstats 'backlog wait' will tell you the time spent waiting for
> an RPC slot after obtaining the NFSv4 session slot.
> 
> The mountstats 'RTT' will give you the time spend waiting for the RPC
> request to be received, processed and replied to by the server.
> 
> Finally, the mountstats also tell you average per-op bytes sent/bytes
> received.
> 
> IOW: The mountstats really gives you almost all the information you
> need here, particularly if you use it in the 'interval reporting' mode.
> The only thing it does not tell you is whether or not the NFSv4 session
> slot table is full (which is why you want the tracepoint).
> 
> > I guess monitoring how much of the time that the client has no free
> > slots might give hints about the first.  If there are always free
> > slots,
> > the first case cannot be the problem.
> > 
> > With NFSv3, the slot management happened at the RPC layer and there
> > were
> > several queues (RPC_PRIORITY_LOW/NORMAL/HIGH/PRIVILEGED) where requests
> > could wait for a free slot.  Since we gained dynamic slot allocation -
> > up to 65536 by default - I wonder if that has much effect any more.
> > 
> > For NFSv4.1+ the slot management is at the NFS level.  The server sets
> > a
> > maximum which defaults to (maybe is limited to) 1024 by the Linux
> > server.
> > So there are always free rpc slots.
> > The Linux client only has a single queue for each slot table, and I
> > think there is one slot table for the forward channel of a session.
> > So it seems we no longer get any priority management (sync writes used
> > to get priority over async writes).
> > 
> > Increasing the number of slots advertised by the server might be
> > interesting.  It is unlikely to fix anything, but it might move the
> > bottle-neck.
> > 
> > Decreasing the maximum of number of tcp slots might also be interesting
> > (below the number of NFS slots at least). 
> > That would allow the RPC priority infrastructure to work, and if the
> > large-file writes are async, they might gets slowed down.
> > 
> > If the problem is in the TCP stream (which is possible if the relevant
> > network buffers are bloated), then you'd really need multiple TCP
> > streams
> > (which can certainly improve throughput in some cases).  That is what
> > nconnect give you.  nconnect does minimal balancing.  It general it
> > will
> > round-robin, but if the number of requests (not bytes) queued on one
> > socket is below average, that socket is likely to get the next request.
> 
> It's not round-robin. Transports are allocated to a new RPC request
> based on a measure of their queue length in order to skip over those
> that show signs of above average congestion.
> 
> > So just adding more connections with nconnect is unlikely to help. 
> > You
> > would need to add a policy engine (struct rpc_xpr_iter_ops) which
> > reserves some connections for small requests.  That should be fairly
> > easy to write a proof-of-concept for.
> 
> Ideally we would want to tie into cgroups as the control mechanism so
> that NFS can be treated like any other I/O resource.
> 
> > 
> > NeilBrown
> > 
> > 
> > > 
> > > Would it be worth experimenting with giving some sort of advantage
> > > to
> > > readers?  (E.g., reserving a few slots for reads and getattrs and
> > > such?)
> > > 
> > > --b.
> > > 
> > > > It's easy to test the case of entirely seperate state & tcp
> > > > connections.
> > > > 
> > > > If we want to test with a shared connection but separate slots I
> > > > guess
> > > > we'd need to create a separate session for each nfs4_server, and
> > > > a lot
> > > > of functions that currently take an nfs4_client would need to
> > > > take an
> > > > nfs4_server?
> > > > 
> > > > --b.
> > > 
> > > 
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
> 
>