On 3/12/20 11:33 PM, Shinichiro Kawasaki wrote:
> 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.
Thanks for testing, it'll go into 5.6.
--
Jens Axboe
