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

[Kanchan Joshi <joshi.k@samsung.com>]

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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.

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