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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 12/16/2021 6:36 PM, Jens Axboe wrote:
> On 12/16/21 9:34 AM, Max Gurtovoy wrote:
>> On 12/16/2021 6:25 PM, Jens Axboe wrote:
>>> On 12/16/21 9:19 AM, Max Gurtovoy wrote:
>>>> 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.
>>> The plug is in the stack of the task, so that condition can never
>>> happen. If the application originally asks for 32 but then only submits
>>> 28, then once that last one is submitted the plug is flushed and
>>> requests are issued.
>> So if I'm running fio with --iodepth=28 what will plug do ? send batches
>> of 28 ? or 1 by 1 ?
> --iodepth just controls the overall depth, the batch submit count
> dictates what happens further down. If you run queue depth 28 and submit
> one at the time, then you'll get one at the time further down too. Hence
> the batching is directly driven by what the application is already
> doing.

I see. Thanks for the explanation.

So it works only for io_uring based applications ?

Don't you think it will be a good idea to not depend on applications and 
batch according to some kernel mechanism ?

Wait till X requests or Y usecs (first condition to be fulfilled) before 
submitting the batch to LLD.

Like we do with adaptive completion coalescing/moderation for capable 
devices.


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