Re: [PATCH 00/14] introduce the BFQ-v0 I/O scheduler as an extra scheduler

[Jens Axboe]

On 10/27/2016 11:32 AM, Ulf Hansson wrote:
> [...]
>
> > 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.
>
> Jens, I think you are wrong here and let me try to elaborate on why.
>
> 1)
> We already have legacy schedulers like CFQ, DEADLINE, etc - and most
> block device drivers are still using the legacy blk interface.

I don't think that's an accurate statement. In terms of coverage, most
drivers do support blk-mq. Anything SCSI, nvme, virtio-blk, SATA runs on
(or can run on) top of blk-mq.

> To be able to remove the legacy blk layer, all block device drivers
> must be converted to blkmq - of course.

That's a given.

> So to reach that goal, we will not only need to evolve blkmq to allow
> scheduling (at least for single queue devices), but we also need to
> convert *all* block device drivers to blkmq. For sure this will take
> *years* and not months.

Correct.

> More important, when the transition to blkmq has been completed, then
> there is absolutely no difference (from effort point of view) in
> removing the legacy blk layer - no matter if we have BFQ in there or
> not.
>
> I do understand if you have concern from maintenance point of view, as
> I assume you would rather focus on evolving blkmq, than care about
> legacy blk code. So, would it help if Paolo volunteers to maintain the
> BFQ code in the meantime?

We're obviously still maintaining the legacy IO path. But we don't want
to actively develop it, and we haven't, for a long time.

And Paolo maintaining it is a strict requirement for inclusion, legacy
or blk-mq aside. That would go for both. I'd never accept a major
feature from an individual or company if they weren't willing and
capable of maintaining it. Throwing submissions over the wall is not
viable.

> 2)
> While we work on evolving blkmq and convert block device drivers to
> it, BFQ could as a separate legacy scheduler, help *lots* of Linux
> users to get a significant improved experience. Should we really
> prevent them from that? I think you block maintainer guys, really need
> to consider this fact.

You still seem to be basing that assumption on the notion that we have
to convert tons of drivers for BFQ to make sense under the blk-mq
umbrella. That's not the case.

> 3)
> While we work on scheduling in blkmq (at least for single queue
> devices), it's of course important that we set high goals. Having BFQ
> (and the other schedulers) in the legacy blk, provides a good
> reference for what we could aim for.

Sure, but you don't need BFQ to be included in the kernel for that.

> > 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.
>
> That's really great news!
>
> I hope we get a possibility to meet and discuss the plans for this at
> Kernel summit/Linux Plumbers the next week!

I'll be there.

--
Jens Axboe