linux-kernel - Re: [BUG] How to get real-time priority using idle priority

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Message-ID: <20090114053106.GA10410@elte.hu>
Date:	Wed, 14 Jan 2009 06:31:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Galbraith <efault@....de>
Cc:	Brian Rogers <brian@...w.org>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [BUG] How to get real-time priority using idle priority


* Mike Galbraith <efault@....de> wrote:

> On Tue, 2009-01-13 at 03:58 +0100, Mike Galbraith wrote:
> > On Mon, 2009-01-12 at 17:05 -0800, Brian Rogers wrote:
> 
> > > I'll try Mike's "more complete" patch on top of 2.6.29-rc1 and see what 
> > > that does.
> > 
> > Don't bother.  I just tried a SCHED_IDLE make -j8 and had character
> > repeats while typing.  Must be another spot.
> 
> Hrmph, what an annoying problem.  The below works pretty well, but 
> _still_ has latency problems in some circumstances.
> 
> The more I look at this, the more I think these guys _really_ want to be 
> a separate class.  The problem is the incredible rate of min_vruntime 
> advancement creating absurdly huge spreads.
> 
> Hm, maybe I could advance min_vruntime at nice 0 when these guys are 
> running, only advance their vruntime at warp 512, but that seems awfully 
> hackish.  If they were a separate class, they could use the full nice 
> spectrum instead of being merely mega-nice.

A separate class has its own set of problems: starvation, etc.

What about increasing the weight of SCHED_IDLE tasks from 1 to 2 or 3? 
That still makes them mega-nice (which is more than enough), but should 
make the math a lot less borderline.

	Ingo
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