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

Max Gurtovoy
2021-12-20T10:12:16Z

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..