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

[Jens Axboe <axboe@kernel.dk>]

On 12/20/21 11:48 AM, Max Gurtovoy wrote:
> 
> On 12/20/2021 6:34 PM, Jens Axboe wrote:
>> On 12/20/21 8:29 AM, Max Gurtovoy wrote:
>>> 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 ?
>> I've only tested with nvme pci which does have it, but I don't think so.
>> Unless there's some check somewhere that makes it necessary. Can you
>> share the patch you're currently using on top?
> 
> The attached POC patches apply cleanly on block/for-next branch

Looks reasonable to me from a quick glance. Not sure why you're not
seeing it hit, maybe try and instrument
block/blk-mq.c:blk_mq_flush_plug_list() and find out why it isn't being
called? As mentioned, no elevator or shared tags, should work for
anything else basically.

-- 
Jens Axboe