[openib-general] Re: [PATCH] [RFC] - example user mode rdmaping/pongprogram using CMA

[Steve Wise]

On Wed, 2006-02-08 at 19:10 +0200, Michael S. Tsirkin wrote:
> Quoting r. Sean Hefty <sean.hefty at intel.com>:
> > Subject: RE: [openib-general] Re: [PATCH] [RFC] - example user mode rdmaping/pongprogram using CMA
> > 
> > >Steve, looks like you have at most a single receive work request posted at the
> > >receive workqueue at all times.
> > >If true, this is *really* not a good idea, performance-wise, even if you
> > >actually have at most 1 packet in flight.
> > 
> > Can you provide some more details on this?
> 
> See 9.7.7.2 end-to-end (message level) flow control
> 

I just read this section in the 1.2 version of the spec, and I still
don't understand what the issue really is?  9.7.7.2 talks about IBA
doing flow control based on the RECV WQEs posted. rping always ensures
that there is a RECV posted before the peer can send.  This is ensured
by the rping protocol itself (see the comment at the front of rping.c
describing the ping loop).

I'm only ever sending one outstanding message via SEND/RECV.  I would
rather post exactly what is needed, than post some number of RECVs "just
to be safe".  Sorry if I'm being dense.  What am I missing here?

Steve.