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

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Message-Id: <AAB0780B-4C8B-4ECD-AC49-59FC872BCF77@linaro.org>
Date:	Sat, 16 Apr 2016 08:03:51 +0200
From:	Paolo Valente <paolo.valente@...aro.org>
To:	Tejun Heo <tj@...nel.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

Hi

Il giorno 16/apr/2016, alle ore 00:45, Tejun Heo <tj@...nel.org> ha scritto:

> Hello, Paolo.
> 
> On Sat, Apr 16, 2016 at 12:08:44AM +0200, Paolo Valente wrote:
>> Maybe the source of confusion is the fact that a simple sector-based,
>> proportional share scheduler always distributes total bandwidth
>> according to weights. The catch is the additional BFQ rule: random
>> workloads get only time isolation, and are charged for full budgets,
>> so as to not affect the schedule of quasi-sequential workloads. So,
>> the correct claim for BFQ is that it distributes total bandwidth
>> according to weights (only) when all competing workloads are
>> quasi-sequential. If some workloads are random, then these workloads
>> are just time scheduled. This does break proportional-share bandwidth
>> distribution with mixed workloads, but, much more importantly, saves
>> both total throughput and individual bandwidths of quasi-sequential
>> workloads.
>> 
>> We could then check whether I did succeed in tuning timeouts and
>> budgets so as to achieve the best tradeoffs. But this is probably a
>> second-order problem as of now.
> 
> Ah, I see.  Yeah, that clears it up for me.

Very glad to hear that!

>  I'm gonna play with
> cgroup settings and see how it actually behaves.
> 

You have already done it two months ago, and ... found a bug!

https://lkml.org/lkml/2016/2/11/731

I will try to spot and fix it (plus the other issues you have
reported), and hopefully get back in a few days with a revised version
of the patchset.

Thanks,
Paolo

> Thanks for your patience. :)
> 
> -- 
> tejun