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

[Michael Krause]

At 11:35 AM 2/8/2006, Steve Wise wrote:

> > >
> > > 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.
> > >
> >
> > As far as I know, the credits are only updated by the ACK messages.
> > If there is a single work request outstanding on the RQ,
> > the ACK of the SEND message will have the credit field value 0
> > (since exactly one receive WR was outstanding, and that is now consumed).
> >
> > As a result the remote side withh "think" that there are no
> > receive WQEs and will slow down (what spec refers to as limited WQE).
>
>Oh.  I understand now.  This is an issue with only 1 RQ WQE posted and
>how IB tries to inform the peer transport of the WQE count.  For iWARP,
>none of this transport-level flow control happens (and I'm more familiar
>with iWARP than IB).

For iWARP, we decided to not implement application receiver based flow 
control due to two items:TCP provides transport-level flow control (IB does 
not provide the equivalent per se) and upon examination of the majority of 
the ULP, they exchange and track the number of receive buffers allowed to 
be processed thus there is no need to replicate this in iWARP.  There are 
some subtleties as well between a message-based transport and a byte stream 
such as TCP that go into the equation but these are not that important for 
most application writers to deal with.

Mike 
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