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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

On 12/20/2021 4:19 PM, Jens Axboe wrote:
> On 12/20/21 3:11 AM, Max Gurtovoy wrote:
>> On 12/19/2021 4:48 PM, Jens Axboe wrote:
>>> 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.
>> I used your block/for-next branch and implemented queue_rqs in NVMe/RDMA
>> but it was never called using the t/io_uring test nor fio with
>> iodepth_batch=32 flag with io_uring engine.
>>
>> Any idea what might be the issue ?
>>
>> I installed fio from sources..
> The two main restrictions right now are a scheduler and shared tags, are
> you using any of those?

No.

But maybe I'm missing the .commit_rqs callback. is it mandatory for this 
feature ?