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

Jens Axboe
2021-12-16T15:48:09Z

On 12/16/21 6:06 AM, Max Gurtovoy wrote:
>
> On 12/16/2021 11:08 AM, Christoph Hellwig wrote:
>> On Wed, Dec 15, 2021 at 09:24:21AM -0700, Jens Axboe wrote:
>>> + 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 :-)

>> 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