Re: [nfsv4] questions about flow control

I'd be happy to discuss it with you, and you and I may have
done so in the past, regarding the approach made by SMB Direct.
This 2012 presentation* discusses some of that design, and
it's perhaps my suggestion to separate the credit protocol
from the credit policy. The protocol is the easy part, honestly.

I'd advise against using the methods in the ATM paper you
cite, at least on their face. These crediting approaches are
largely attempting to meter available bandwidth. They also
are for protocols which are tolerant of loss. This is a very
different goal from the RDMA crediting required to provide a
reliable stream of requests and responses.

Tom.

* 
https://www.snia.org/sites/default/orig/SDC2012/presentations/Revisions/TomTalpeyKramer-High_Performance__File.pdf

On 4/29/2021 2:16 PM, Chuck Lever III wrote:
> Howdy-
> 
> We're working on prototyping RPC/RDMA version two. As many of
> you know, RPC/RDMA uses credit-based flow control.
> 
> I've presented to the WG before on the kinds of improvements
> to credit accounting we need to make over version one of
> RPC/RDMA in order to support control plane operations and
> message continuation -- cases where we no longer have
> perfectly symmetrical Call/Reply pairing.
> 
> I'm looking at Section 4.2.1.1 of draft-ietf-nfsv4-rpcrdma-version-two
> as it is currently constructed and I'm finding it ...
> underwhelming.
> 
> I'm thinking of replacing it with something more akin to the
> original forms of credit-based flow control, as described in
> Chapter 4 of:
> 
> https://dl.acm.org/doi/pdf/10.1145/190314.190324
> 
> and implemented in the form of Chapter 5 of that paper. The
> rdma_credits field would be filled in with the sender's Vr,
> in both directions, and N2 + N3 would be the credit limit. We
> would need to add some kind of "reset credit accounting"
> message as well.
> 
> I'm not feeling confident about this choice, however. Does
> anyone know a person or people who could answer some questions
> about flow control design? Or is there a good reference I could
> read to help me understand fundamentals and common pitfalls?
> 
> Many thanks in advance!
> 
> 
> --
> Chuck Lever
> 
> 
> 
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