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

Jens Axboe
2021-12-16T17:16:39Z

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.

> 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.

--
Jens Axboe