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

[Arlin Davis]

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.

-arlin

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