On Mar 12, 2020 / 07:26, Jens Axboe wrote:
> On 3/12/20 3:15 AM, Ming Lei wrote:
> > commit 01e99aeca397 ("blk-mq: insert passthrough request into
> > hctx->dispatch directly") may change to add flush request to the tail
> > of dispatch by applying the 'add_head' parameter of
> > blk_mq_sched_insert_request.
> >
> > Turns out this way causes performance regression on NCQ controller because
> > flush is non-NCQ command, which can't be queued when there is any in-flight
> > NCQ command. When adding flush rq to the front of hctx->dispatch, it is
> > easier to introduce extra time to flush rq's latency compared with adding
> > to the tail of dispatch queue because of S_SCHED_RESTART, then chance of
> > flush merge is increased, and less flush requests may be issued to
> > controller.
> >
> > So always insert flush request to the front of dispatch queue just like
> > before applying commit 01e99aeca397 ("blk-mq: insert passthrough request
> > into hctx->dispatch directly").
>
> Applied, thanks.
Ming, thank you so much for the patch. Using my SMR SATA drive I confirmed it
reduces blktests block/004 runtime as expected. With this patch, the runtime is
like before the commit 01e99aeca397. Good.
--
Best Regards,
Shin'ichiro Kawasaki
