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

[Jens Axboe <axboe@kernel.dk>]

On 12/19/21 5:14 AM, Max Gurtovoy wrote:
> 
> On 12/16/2021 7:16 PM, Jens Axboe wrote:
>> On 12/16/21 9:57 AM, Max Gurtovoy wrote:
>>> 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 ?
>> It's only enabled for io_uring right now, but it's generically available
>> for anyone that wants to use it... Would be trivial to do for aio, and
>> other spots that currently use blk_start_plug() and has an idea of how
>> many IOs will be submitted
> 
> Can you please share an example application (or is it fio patches) that 
> can submit batches ? The same that was used to test this patchset is 
> fine too.
> 
> I would like to test it with our NVMe SNAP controllers and also to 
> develop NVMe/RDMA queue_rqs code and test the perf with it.

You should just be able to use iodepth_batch with fio. For my peak
testing, I use t/io_uring from the fio repo. By default, it'll run QD of
and do batches of 32 for complete and submit. You can just run:

t/io_uring <dev or file>

maybe adding -p0 for IRQ driven rather than polled IO.

-- 
Jens Axboe