linux-kernel - Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

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Date:	Wed, 18 Apr 2007 05:56:42 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Mike Galbraith <efault@....de>
Cc:	Matt Mackall <mpm@...enic.com>,
	William Lee Irwin III <wli@...omorphy.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Con Kolivas <kernel@...ivas.org>, Ingo Molnar <mingo@...e.hu>,
	ck list <ck@....kolivas.org>,
	Bill Huey <billh@...ppy.monkey.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

On Wed, Apr 18, 2007 at 05:45:20AM +0200, Mike Galbraith wrote:
> On Wed, 2007-04-18 at 05:15 +0200, Nick Piggin wrote:
> > On Tue, Apr 17, 2007 at 04:39:54PM -0500, Matt Mackall wrote:
> > > 
> > > I'm a big fan of fairness, but I think it's a bit early to declare it
> > > a mandatory feature. Bounded unfairness is probably something we can
> > > agree on, ie "if we decide to be unfair, no process suffers more than
> > > a factor of x".
> > 
> > I don't know why this would be a useful feature (of course I'm talking
> > about processes at the same nice level). One of the big problems with
> > the current scheduler is that it is unfair in some corner cases. It
> > works OK for most people, but when it breaks down it really hurts. At
> > least if you start with a fair scheduler, you can alter priorities
> > until it satisfies your need... with an unfair one your guess is as
> > good as mine.
> > 
> > So on what basis would you allow unfairness? On the basis that it doesn't
> > seem to harm anyone? It doesn't seem to harm testers?
> 
> Well, there's short term fair and long term fair.  Seems to me a burst
> load having to always merge with a steady stream load using a short term
> fairness yardstick absolutely must 'starve' relative to the steady load,
> so to be long term fair, you have to add some short term unfairness.
> The mainline scheduler is more long term fair (discounting the rather
> obnoxious corner cases).

Oh yes definitely I mean long term fair. I guess it is impossible to be
completely fair so long as we have to timeshare the CPU :)

So a constant delta is fine and unavoidable. But I don't think I agree
with a constant factor: that means you can pick a time where process 1
is allowed an arbitrary T more CPU time than process 2.

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