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@xxxxxx> wrote:
>
> On Tue, Mar 08, 2022 at 08:50:58PM +0530, Kanchan Joshi wrote:
> > From: Jens Axboe <axboe@xxxxxxxxx>
> >
> > 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.
