Selective signalling vs WQE size - was Re: [ofa-general] mthca max_sge value... ugh.

[Talpey, Thomas]

At 07:12 PM 5/16/2008, Roland Dreier wrote:
>if we can't use the "WQE shrinking" feature (because of selective
>signaling in the NFS/RDMA case), and we want to use 32 sge entries, then
>the WQE size 's' will end up a little more than 512 bytes, and the
>wqe_shift will end up as 10.

Can you elaborate on this? The NFS/RDMA client does selective signalling
on its send queue in order to save on interrupts and CQE generation/handling.
Which I always thought was a (very) good approach. Because the RPC
request/response paradigm guarantees an eventual receive completion,
we simply defer (or even completely avoid) this work.

Would that be a bad trade if it takes a WQE management opportunity away
from the provider? It's quite easy to change this in the NFS/RDMA code,
or make it a selectable parameter.

Tom.