linux-kernel - Re: Question about fair schedulers

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Message-Id: <200706231556.36430.info@gnebu.es>
Date:	Sat, 23 Jun 2007 15:56:36 +0200
From:	Alberto Gonzalez <info@...bu.es>
To:	Paolo Ornati <ornati@...twebnet.it>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Question about fair schedulers

On Saturday 23 June 2007, Paolo Ornati wrote:
> But the fact is, the "interactivity estimator" is too fragile, and when
> it fails it can do much damage.
>
>
> Fair scheduler instead:
> 	- are robust
> 	- provide consistent behaviour
> 	- provide good interactivity within the bounds of fairness

Yes, this is what I have concluded from this thread. Trying to guess which 
tasks should have higher priority is not the right approach. It's not a CPU 
scheduler's business to decide about priorities.

> > In my very simple test scenario the mainline scheduler
> > did behave much better.
>
> Of course... because of the two competing processes the
> "interactive" (for you) one needs 60%, that is more than it's 50% fair
> share.
>
> The real solution is to use nice levels so the scheduler doesn't have
> to guess what process is more important.

Yes, this seems to be the right approach from all the opinions I've heard.

> And yes, programs/distributions should set good defaults for you... and
> if they don't, just complain to them  :)

I'm sure they'll do once a fair scheduler goes into mainline :)

I guess what I was missing from the beginning is that "fair" means that the 
scheduler will be fair among tasks that have the same priority, but if a task 
has a higher priority, it _will_ get more CPU. So we'll just have to mark 
applications like video players, audio players or games with a high priority, 
others like encoders or compilers with low priority, and leave the rest 
(browsers, word processors, email readers, etc...) as normal priority. This 
way a fair scheduler would be able to give each task right amount of CPU.

That sounds like a good solution to me.

Thanks,
Alberto.
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