Re: [PATCH 10/17] block: wire-up support for plugging

Jens Axboe
2022-03-23T02:17:57Z

On 3/22/22 8:10 PM, Ming Lei wrote:
> On Tue, Mar 22, 2022 at 07:58:25PM -0600, Jens Axboe wrote:
>> On 3/22/22 7:41 PM, Jens Axboe wrote:
>>> On 3/22/22 7:27 PM, Ming Lei wrote:
>>>> On Mon, Mar 21, 2022 at 12:32:08PM +0530, Kanchan Joshi wrote:
>>>>> On Mon, Mar 14, 2022 at 10:40:53PM +0800, Ming Lei wrote:
>>>>>> On Thu, Mar 10, 2022 at 06:10:08PM +0530, Kanchan Joshi wrote:
>>>>>>> On Thu, Mar 10, 2022 at 2:04 PM Christoph Hellwig <hch@lst.de> wrote:
>>>>>>>>
>>>>>>>> On Tue, Mar 08, 2022 at 08:50:58PM +0530, Kanchan Joshi wrote:
>>>>>>>>> From: Jens Axboe <axboe@kernel.dk>
>>>>>>>>>
>>>>>>>>> Add support to use plugging if it is enabled, else use default path.
>>>>>>>>
>>>>>>>> The subject and this comment don't really explain what is done, and
>>>>>>>> also don't mention at all why it is done.
>>>>>>>
>>>>>>> Missed out, will fix up. But plugging gave a very good hike to IOPS.
>>>>>>
>>>>>> But how does plugging improve IOPS here for passthrough request? Not
>>>>>> see plug->nr_ios is wired to data.nr_tags in blk_mq_alloc_request(),
>>>>>> which is called by nvme_submit_user_cmd().
>>>>>
>>>>> Yes, one tag at a time for each request, but none of the request gets
>>>>> dispatched and instead added to the plug. And when io_uring ends the
>>>>> plug, the whole batch gets dispatched via ->queue_rqs (otherwise it used
>>>>> to be via ->queue_rq, one request at a time).
>>>>>
>>>>> Only .plug impact looks like this on passthru-randread:
>>>>>
>>>>> KIOPS(depth_batch) 1_1 8_2 64_16 128_32
>>>>> Without plug 159 496 784 785
>>>>> With plug 159 525 991 1044
>>>>>
>>>>> Hope it does clarify.
>>>>
>>>> OK, thanks for your confirmation, then the improvement should be from
>>>> batch submission only.
>>>>
>>>> If cached request is enabled, I guess the number could be better.
>>>
>>> Yes, my original test patch pre-dates being able to set a submit count,
>>> it would definitely help improve this case too. The current win is
>>> indeed just from being able to use ->queue_rqs() rather than single
>>> submit.
>>
>> Actually that is already there through io_uring, nothing extra is
>> needed.
>
> I meant in this patchset that plug->nr_ios isn't wired to data.nr_tags in
> blk_mq_alloc_request(), which is called by pt request allocation, so
> looks cached request isn't enabled for pt commands?

My point is that it is, since submission is through io_uring which does
exactly that.

--
Jens Axboe