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

Jens Axboe
2022-03-23T01:41:42Z

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.

--
Jens Axboe