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

Ming Lei
2022-03-23T01:27:49Z

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.

Thanks,
Ming