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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 12/16/2021 5:48 PM, Jens Axboe wrote:
> 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.

I'm saying that the you can wait to the batch_max_count too long and it 
won't be efficient from latency POV.

So it's better to limit the block layar to wait for the first to come: x 
usecs or batch_max_count before issue queue_rqs.

Also, This batch is per HW queue or SW queue or the entire request queue ?

>