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

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Date:	Mon, 02 Jun 2014 11:32:05 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Paolo Valente <paolo.valente@...more.it>,
	Li Zefan <lizefan@...wei.com>,
	Fabio Checconi <fchecconi@...il.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>,
	Paolo Valente <posta_paolo@...oo.it>,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org
Subject: Re: [PATCH RFC - TAKE TWO - 00/12] New version of the BFQ I/O Scheduler

On 06/02/2014 11:24 AM, Tejun Heo wrote:
> Hello, Jens.
> 
> On Mon, Jun 02, 2014 at 08:29:27AM -0600, Jens Axboe wrote:
>> One thing I've neglected to bring up but have been thinking about - we're
>> quickly getting to the point where the old request_fn IO path will become a
>> legacy thing, mostly in maintenance mode. That isn't a problem for morphing
>> bfq and cfq, but it does mean that development efforts in this area would be
>> a lot better spent writing an IO scheduler that fits into the blk-mq
>> framework instead.
> 
> What I'm planning right now is improving blkcg so that it can do both
> proportional and hard limits with high cpu scalability, most likely
> using percpu charge caches.  It probably would be best to roll all
> those into one piece of logic.  I don't think, well at least hope,
> that we'd need multiple modular scheduler / blkcg implementations for
> blk-mq and both can be served by built-in scheduling logic.
> Regardless of device speed, we'd need some form of fairness
> enforcement after all.

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.

-- 
Jens Axboe

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