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

Ming Lei
2022-03-23T02:10:35Z

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?

Thanks,
Ming