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

[Tom Tucker]

On Wed, 2006-02-08 at 23:20 -0800, Sean Hefty wrote:
> >Hmm.  Can you put a number on how much better RDMA write with
> >immediate is on current HCA hardware?  How does using the underlying
> >OpenIB verbs ability to post a list of work requests compare (ie
> >posting an RDMA write followed by a send in one verbs call)?
> >Maybe "post multiple" is a better direction for DAT.
> 
> A "post multiple" call as a general API makes sense, but I think that's a
> separate issue.
> 
> Given that IB provides true immediate data with RDMA writes, a way should be
> available to make use of it.  I don't know what the performance numbers between
> using a write with immediate versus a write followed by a send, but I don't
> think that anyone could argue that the write with immediate wouldn't perform
> better.
> 
> To me, the question is whether write with immediate is supported as a transport
> specific extension, which was Arlin's original patch, or through some standard
> API.  The attempt to make the API standard, so that iWarp could emulate it
> (poorly in my view), is what appears to be driving the disagreements.
> 
> It also appears to me that the decisions are coming down to one of the
> following.  If iWarp can emulate write with immediate, then a generic API should
> be used.  

This opens Pandora's box. Should iWARP also emulate ATOMICs? Which
should be emulated and which should not ... What are the criteria for
deciding? 

> If iWarp cannot properly emulate write with immediate, then the API
> should be transport specific.  

It should be transport specific because it is a transport specific
feature. Although -- in this case -- it could but implemented in iWARP
in my view it _should_ not. 

> It's curious to me that in both cases, iWarp is
> driving the API decision and design for something that is an IB specific
> feature.

Huh?

> 
> - Sean
> 
> 
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