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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

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.

> .
>
>> Don't you think it will be a good idea to not depend on applications and
>> batch according to some kernel mechanism ?
>>
>> Wait till X requests or Y usecs (first condition to be fulfilled) before
>> submitting the batch to LLD.
>>
>> Like we do with adaptive completion coalescing/moderation for capable
>> devices.
> This is how plugging used to work way back in the day. The problem is
> that you then introduce per-device state, which can cause contention.
> That's why the plug is a pure stack based entity now.
>