[PATCH RFC] nvme-rdma: support devices with queue size < 32

[dledford@redhat.com (Doug Ledford)]

On 3/29/17 11:39 AM, Sagi Grimberg wrote:
>
>>> For each I/O we post up to 2 work requests, 1 for memory registration
>>> and 1 for sending an I/O request (and 1 for local invalidation if the
>>> target doesn't do it for us, but that is not the case here). So if our
>>> queue depth is X, we size our completion queue to be X*3, and we need
>>> to make sure we signal every (X*3)/2.
>>
>> ??? If your SQ is X and your CQ is X*3 you need to signal at X/2.
>
> Sorry, I confused SQ with CQ (which made it even more confusing..)
>
> Our application queue-depth is X, we size our SQ to be X*3
> (send+reg+inv), we size our RQ to be X (resp) and our CQ to be
> X*4 (SQ+RQ).
>
> So we should signal every (X*3)/2

You say above "we post *up to* 2 work requests", unless you wish to 
change that to "we always post at least 2 work requests per queue 
entry", Jason is right, your frequency of signaling needs to be X/2 
regardless of your CQ size, you need the signaling to control the queue 
depth tracking.