Re: [PATCH V2 14/20] blk-mq-sched: improve IO scheduling on SCSI devcie

Ming Lei
2017-08-24T07:14:43Z

On Tue, Aug 22, 2017 at 08:51:32PM +0000, Bart Van Assche wrote:
> On Sat, 2017-08-05 at 14:56 +0800, Ming Lei wrote:
> > This patch introduces per-request_queue dispatch
> > list for this purpose, and only when all requests
> > in this list are dispatched out successfully, we
> > can restart to dequeue request from sw/scheduler
> > queue and dispath it to lld.
>
> Wasn't one of the design goals of blk-mq to avoid a per-request_queue list?

Yes, but why does scsi device have per-request_queue depth?

Anyway this single dispatch list won't hurt devices which doesn't have
per-request_queue depth.

For scsi device which has q->queue_depth, if one hctx is stuck, all
hctxs are stuck actually, the single dispatch list just fits this
kind of use case, doesn't it?

--
Ming