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

Jens Axboe
2021-12-21T15:23:40Z

On 12/21/21 3:20 AM, Max Gurtovoy wrote:
>
> On 12/20/2021 8:58 PM, Jens Axboe wrote:
>> 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.
>
> Yes. I saw that the blk layer converted the original non-shared tagset
> of NVMe/RDMA to a shared one because of the nvmf connect request queue
> that is using the same tagset (uses only the reserved tag).
>
> So I guess this is the reason that the I couldn't reach the new code of
> queue_rqs.
>
> The question is how we can overcome this ?

Do we need to mark it shared for just the reserved tags? I wouldn't
think so...

--
Jens Axboe