On Wed 26-10-16 10:12:38, Jens Axboe wrote: > On 10/26/2016 10:04 AM, Paolo Valente wrote: > > > >>Il giorno 26 ott 2016, alle ore 17:32, Jens Axboe <axboe@xxxxxxxxx> ha scritto: > >> > >>On 10/26/2016 09:29 AM, Christoph Hellwig wrote: > >>>On Wed, Oct 26, 2016 at 05:13:07PM +0200, Arnd Bergmann wrote: > >>>>The question to ask first is whether to actually have pluggable > >>>>schedulers on blk-mq at all, or just have one that is meant to > >>>>do the right thing in every case (and possibly can be bypassed > >>>>completely). > >>> > >>>That would be my preference. Have a BFQ-variant for blk-mq as an > >>>option (default to off unless opted in by the driver or user), and > >>>not other scheduler for blk-mq. Don't bother with bfq for non > >>>blk-mq. It's not like there is any advantage in the legacy-request > >>>device even for slow devices, except for the option of having I/O > >>>scheduling. > >> > >>It's the only right way forward. blk-mq might not offer any substantial > >>advantages to rotating storage, but with scheduling, it won't offer a > >>downside either. And it'll take us towards the real goal, which is to > >>have just one IO path. > > > >ok > > > >>Adding a new scheduler for the legacy IO path > >>makes no sense. > > > >I would fully agree if effective and stable I/O scheduling would be > >available in blk-mq in one or two months. But I guess that it will > >take at least one year optimistically, given the current status of the > >needed infrastructure, and given the great difficulties of doing > >effective scheduling at the high parallelism and extreme target speeds > >of blk-mq. Of course, this holds true unless little clever scheduling > >is performed. > > > >So, what's the point in forcing a lot of users wait another year or > >more, for a solution that has yet to be even defined, while they could > >enjoy a much better system, and then switch an even better system when > >scheduling is ready in blk-mq too? > > That same argument could have been made 2 years ago. Saying no to a new > scheduler for the legacy framework goes back roughly that long. We could > have had BFQ for mq NOW, if we didn't keep coming back to this very > point. > > I'm hesistant to add a new scheduler because it's very easy to add, very > difficult to get rid of. If we do add BFQ as a legacy scheduler now, > it'll take us years and years to get rid of it again. We should be > moving towards LESS moving parts in the legacy path, not more. > > We can keep having this discussion every few years, but I think we'd > both prefer to make some actual progress here. It's perfectly fine to > add an interface for a single queue interface for an IO scheduler for > blk-mq, since we don't care too much about scalability there. And that > won't take years, that should be a few weeks. Retrofitting BFQ on top of > that should not be hard either. That can co-exist with a real multiqueue > scheduler as well, something that's geared towards some fairness for > faster devices. OK, so some solution like having a variant of blk_sq_make_request() that will consume requests, do IO scheduling decisions on them, and feed them into the HW queue is it sees fit would be acceptable? That will provide the IO scheduler a global view that it needs for complex scheduling decisions so it should indeed be relatively easy to port BFQ to work like that. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
