linux-kernel - Re: [PATCH RFC 09/22] block, cfq: replace CFQ with the BFQ-v0 I/O scheduler

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Date:	Fri, 15 Apr 2016 11:08:35 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Paolo Valente <paolo.valente@...aro.org>
Cc:	Jens Axboe <axboe@...nel.dk>, Fabio Checconi <fchecconi@...il.com>,
	Arianna Avanzini <avanzini.arianna@...il.com>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH RFC 09/22] block, cfq: replace CFQ with the BFQ-v0 I/O
 scheduler

Hello, Paolo.

On Fri, Apr 15, 2016 at 04:20:44PM +0200, Paolo Valente wrote:
> > It's actually a lot more difficult to answer that with bandwidth
> > scheduling.  Let's say cgroup A has 50% of disk time.  Sure, there are
> > inaccuracies, but it should be able to get close to the ballpark -
> > let's be lax and say between 30% and 45% of raw sequential bandwidth.
> > It isn't ideal but now imagine bandwidth based scheduling.  Depending
> > on what the others are doing, it may get 5% or even lower of the raw
> > sequential bandwidth.  It isn't isolating anything.
> 
> Definitely. Nevertheless my point is still about the same: we have to
> consider one system at a time. If the workload of the system is highly
> variable and completely unpredictable, then it is hard to provide any
> bandwidth guarantee with any solution.

I don't think that is true with time based scheduling.  If you
allocate 50% of time, it'll get close to 50% of IO time which
translates to bandwidth which is lower than 50% but still in the
ballpark.  That is very different from "we can't guarantee anything if
the other workloads are highly variable".

So, I get that for a lot of workload, especially interactive ones, IO
patterns are quasi-sequential and bw based scheduling is beneficial
and we don't care that much about fairness in general; however, it's
problematic that it would make the behavior of proportional control
quite surprising.

> > As I wrote before, as fairness isn't that important for normal
> > scheduling, if empirical data show that bandwidth based scheduling is
> > beneficial for most common workloads, that's awesome especially given
> > that CFQ has plenty of issues.  I don't think cgroup case is workable
> > as currently implemented tho.
> 
> I was thinking about some solution to achieve both goals. An option is
> probably to let BFQ work in a double mode: sector-based within groups
> and time-based among groups. However, I find it a little messy and
> confusing.
> 
> Other ideas/solutions? I have no better proposal at the moment :(

No idea.  I don't think isolation could work without time based
scheduling at some level tho. :(

Thanks.

-- 
tejun