Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler

[Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>]

On 06/02/2014 11:42 AM, Tejun Heo wrote:
> Hello, Jens.
> 
> On Mon, Jun 02, 2014 at 11:32:05AM -0600, Jens Axboe wrote:
>> For things like blkcg, I agree, it should be able to be common code and
>> reusable. But there's a need for scheduling beyond that, for people that
>> don't use control groups (ie most...). And it'd be hard to retrofit cfq
>> into blk-mq, without rewriting it. I don't believe we need anything this
>> fancy for blk-mq, hopefully. At least having simple deadline scheduling
>> would be Good Enough for the foreseeable future.
> 
> Heh, looks like we're miscommunicating.  I don't think anything with
> the level of complexity of cfq is realistic for high-iops devices.  It
> has already become a liability for SATA ssds after all.  My suggestion
> is that as hierarchical scheduling tends to be logical extension of
> flat scheduling, it probably would make sense to implement both
> scheduling logics in the same framework as in the cpu scheduler or (to
> a lesser extent) cfq.  So, a new blk-mq scheduler which can work in
> hierarchical mode if blkcg is in active use.

But blk-mq will potentially drive anything, so it might not be out of
the question with a more expensive scheduling variant, if it makes any
sense to do of course. At least until there's no more rotating stuff out
there :-). But it's not a priority at all to me yet. As long as we have
coexisting IO paths, it'd be trivial to select the needed one based on
the device characteristics.

> One part I was wondering about is whether we'd need to continue the
> modular multiple implementation mechanism.  For rotating disks, for
> various reasons including some historical ones, we ended up with
> multiple ioscheds and somewhat uglily layered blkcg implementations.
> Given that the expected characteristics of blk-mq devices are more
> consistent, it could be reasonable to stick with single iops and/or
> bandwidth scheme.

I hope not to do that. I just want something sane and simple (like a
deadline scheduler), nothing more.

-- 
Jens Axboe