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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 12/16/2021 6:05 PM, Jens Axboe wrote:
> On 12/16/21 9:00 AM, Max Gurtovoy wrote:
>> 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.
> There's no waiting specifically for this, it's just based on the plug.
> We just won't do more than 32 in that plug. This is really just an
> artifact of the plugging, and if that should be limited based on "max of
> 32 or xx time", then that should be done there.
>
> But in general I think it's saner and enough to just limit the total
> size. If we spend more than xx usec building up the plug list, we're
> doing something horribly wrong. That really should not happen with 32
> requests, and we'll never eg wait on requests if we're out of tags. That
> will result in a plug flush to begin with.

I'm not aware of the plug. I hope to get to it soon.

My concern is if the user application submitted only 28 requests and 
then you'll wait forever ? or for very long time.

I guess not, but I'm asking how do you know how to batch and when to 
stop in case 32 commands won't arrive anytime soon.

>
>> Also, This batch is per HW queue or SW queue or the entire request queue ?
> It's per submitter, so whatever the submitter ends up queueing IO
> against. In general it'll be per-queue.

struct request_queue ?

I think the best is to batch per struct blk_mq_hw_ctx.

I see that you check this in the nvme_pci driver but shouldn't it go to 
the block layer ?

>