[dat-discussions] [openib-general] [RFC] DAT2.0immediatedataproposal

[Michael Krause]

At 09:16 PM 2/6/2006, Sean Hefty wrote:
> >The requirement is to provide an API that supports RDMA writes with 
> immediate
> >data.  A send that follows an RDMA write is not immediate data, and the API
> >should not be constructed around trying to make it so.
>
>To be clear, I believe that write with immediate should be part of the normal
>APIs, rather than an extension, but should be designed around those 
>devices that
>provide it natively.

One thing to keep in mind is that the IBTA workgroup responsible for the 
transport wanted to eliminate immediate data support entirely but it was 
retained solely to enable VIA application migration (even though the 
application base was quite small).  If that requirement could have been 
eliminated, then it would have been gone in a heart beat.  Given a 
RDMA-WRITE followed by a SEND provides the same application semantics based 
on the use models, iWARP chose not to support immediate data.

So, here we have a long discussion on attempting to perpetuate a concept 
that is not universal across transports and was deemed to have minimal 
value that most wanted to see removed from the architecture.  One has to 
question the value of trying to develop any API / software to support 
immediate data instead of just enabling the preferred method which is RDMA 
WRITE - SEND.  I agree with those who have contended that this is difficult 
to do in a general purpose fashion.  When all of this is taken into 
account, it seems the only good engineering answer is to eliminate 
immediate data support by the software and focused on the method that works 
across all interconnects.

Mike 
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