Re: [PATCH 4/4] nvme: add support for mq_ops->queue_rqs()

On 12/16/21 6:06 AM, Max Gurtovoy wrote:

On 12/16/2021 11:08 AM, Christoph Hellwig wrote:

quoted

On Wed, Dec 15, 2021 at 09:24:21AM -0700, Jens Axboe wrote:

quoted

+	spin_lock(&nvmeq->sq_lock);
+	while (!rq_list_empty(*rqlist)) {
+		struct request *req = rq_list_pop(rqlist);
+		struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
+
+		memcpy(nvmeq->sq_cmds + (nvmeq->sq_tail << nvmeq->sqes),
+				absolute_pointer(&iod->cmd), sizeof(iod->cmd));
+		if (++nvmeq->sq_tail == nvmeq->q_depth)
+			nvmeq->sq_tail = 0;

So this doesn't even use the new helper added in patch 2?  I think this
should call nvme_sq_copy_cmd().

I also noticed that.

So need to decide if to open code it or use the helper function.

Inline helper sounds reasonable if you have 3 places that will use it.

Yes agree, that's been my stance too :-)

quoted

The rest looks identical to the incremental patch I posted, so I guess
the performance degration measured on the first try was a measurement
error?

giving 1 dbr for a batch of N commands sounds good idea. Also for RDMA host.

But how do you moderate it ? what is the batch_sz <--> time_to_wait 
algorithm ?

The batching is naturally limited at BLK_MAX_REQUEST_COUNT, which is 32
in total. I do agree that if we ever made it much larger, then we might
want to cap it differently. But 32 seems like a pretty reasonable number
to get enough gain from the batching done in various areas, while still
not making it so large that we have a potential latency issue. That
batch count is already used consistently for other items too (like tag
allocation), so it's not specific to just this one case.

-- 
Jens Axboe