[dat-discussions] [openib-general] [RFC] DAT 2.0 immediate data proposal

[Sean Hefty]

Davis, Arlin R wrote:
> “Applications need an optimized mechanism to notify the receiving end 
> that RDMA write data has completed beyond the two operation method 
> currently used (RDMA write followed by message send). This new RDMA 
> write feature will support 4-bytes of inline data that will be sent 

Is there any reason to restrict the size of the immediate data?  Could you 
define the API such that the size is variable?  I.e. the provider can simply 
give the immediate data size, with 0 indicating that it is not supported.

> It should avoid 
> any latency penalties normally associated with a two operation method. 

I would state this as a requirement.  A write followed by a send should be 
pushed to the application, since they may be able to provide additional 
optimizations (such as combining operations) beyond what a provider could.

> The initiating side must expose a 4-byte immediate data parameter for 
> the application to set the inline data. The receiving side must provide 
> a mechanism to accept the 4-byte immediate data. On the receiving side, 
> the write with immediate completion notification is indicated through a 
> receive completion. It is the responsibility of the provider to identify 
> to the application 4-byte immediate data from a normal 4-byte send 
> message. The inline byte ordering is application specific.”

Requirements look good to me.

- Sean