[ofiwg] send/recv "credits"

Jason Gunthorpe jgunthorpe at obsidianresearch.com
Sun Sep 28 13:04:23 PDT 2014

On Sat, Sep 27, 2014 at 04:59:07PM +0000, Hefty, Sean wrote:
> </on soapbox> Honestly, I think an app designed for EAGAIN will be
> the best performing.  A credit based scheme introduces more
> branches, additional memory reads and writes, and arithmetic
> operations.  EAGAIN optimizes for a common case of having available
> queue space.  </off soapbox>

Every app I've built uses a single arithmetic to combine the sendq,
completionq, and remote recvq limit tests:

 if (have_send < can_send) // on a ring

and can_send is updated during completion processing.

AFAICS you cannot eliminate the if, except possibly, in special cases
like non-event-driven applications, and applications relying on RNR
Acks. Otherwise you always need a test of some sort.

I also tend to supress interrupts on all send completions except ones
that are the last-to-insert as an interrupt mitigation strategy. Not
sure how you do that with EAGAIN.

Also, what are the semantics of a full completion Q? Does iwarp blow
up if you have a recv wqe but no space for a cqe?

I've always designed to avoid full completion q conditions - sendq
credit is based on when the sqe is recycled back via the completion q,
and recvq credit is computed considering available space in the compq
after considering sendq usage.


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