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

[Michael Krause]

At 03:36 PM 2/8/2006, Arlin Davis wrote:
>Roland Dreier wrote:
>
>>    Michael> So, here we have a long discussion on attempting to
>>    Michael> perpetuate a concept that is not universal across
>>    Michael> transports and was deemed to have minimal value that most
>>    Michael> wanted to see removed from the architecture.
>>
>>But this discussion is being driven by an application developer who
>>does see value in immediate data.
>>
>>Arlin, can you quantify the benefit you see from RDMA write with
>>immediate vs. RDMA write followed by a send?
>>
>>
>We need speed and simplicity.
>
>A very latency sensitive application that requires immediate notification 
>of RDMA write completion on the remote node without ANY latency penalties 
>associated with combining operations, HCA priority rules across QPs, wire 
>congestion, etc. An application that has no requirement for messaging 
>outside of remote rdma write completion notifications. The application 
>would not have to register and manage additional message buffers on either 
>side, we can just size the queues accordingly and post zero byte messages. 
>We need something that would be equivelent to setting there polling on the 
>last byte of inbound data. But, since data ordering within an operation is 
>not guaranteed that is not an option. So, rdma with immediate data is the 
>most optimal and simplistic method for indication of RDMA-write completion 
>that we have available today. In fact, I would like to see it increased in 
>size to make it even more useful.

RDMA Write with Immediate is part of the IB Extended Transport Header.  It 
is a fixed-sized quantity and not one subject to change, i.e. increasing 
its size.

Your argument above reinforces that the particular application need is 
IB-specific and thus should not be part of a general API but a 
transport-specific API.   If the application will only operate optimally 
using immediate data, then it is only suitable for an IB fabric.  This 
reinforces the need for a transport-specific API.

Those applications that simply want to enable completion notification when 
a RDMA Write has occurred can use a general purpose API that is 
interconnect independent and whose code is predicated upon a RDMA Write - 
Send set of operations.  This will enable application portability across 
all interconnect types.

Mike 
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