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

[Sagi Grimberg]

>> Thanks, this looks useful.  But wouldn't it be better to do something
>> like queue_size divided by 2 or 4 to get a better refill latency?
>
> That's an interesting question. The max number of requests is already at 3 or 4 times
> of the queue size because of different message types (see Sam's original
> message in 'NVMe RDMA driver: CX4 send queue fills up when nvme queue depth is low').
> I guess it would have inflence on configs with bigger latency.
>
> I would like to have Sagi's view on this as he's the one who has changed that
> part in the iSER initiator in 6df5a128f0fde6315a44e80b30412997147f5efd

Hi Marta,

This looks good indeed. IIRC both for IB and iWARP we need to signal
at least once every send-queue depth (in practice I remember that some
devices need more than once) so maybe it'll be good to have division by
2.

Maybe it'll be better if we do:

static inline bool queue_sig_limit(struct nvme_rdma_queue *queue)
{
	return (++queue->sig_count % (queue->queue_size / 2)) == 0;
}

And lose the hard-coded 32 entirely. Care to test that?