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

Jens Axboe
2022-03-23T01:58:34Z

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.

--
Jens Axboe