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

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Message-ID: <20070417225723.GP11115@waste.org>
Date:	Tue, 17 Apr 2007 17:57:23 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	William Lee Irwin III <wli@...omorphy.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Davide Libenzi <davidel@...ilserver.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Williams <pwil3058@...pond.net.au>,
	Mike Galbraith <efault@....de>,
	Con Kolivas <kernel@...ivas.org>, ck list <ck@....kolivas.org>,
	Bill Huey <billh@...ppy.monkey.org>,
	Linux Kernel Mailing List <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 Tue, Apr 17, 2007 at 03:59:02PM -0700, William Lee Irwin III wrote:
> On Tue, Apr 17, 2007 at 03:32:56PM -0700, William Lee Irwin III wrote:
> >> I'm already working with this as my assumed nice semantics (actually
> >> something with a specific exponential base, suggested in other emails)
> >> until others start saying they want something different and agree.
> 
> On Tue, Apr 17, 2007 at 05:39:09PM -0500, Matt Mackall wrote:
> > Good. This has a couple nice mathematical properties, including
> > "bounded unfairness" which I mentioned earlier. What base are you
> > looking at?
> 
> I'm working with the following suggestion:
> 
> 
> On Tue, Apr 17, 2007 at 09:07:49AM -0400, James Bruce wrote:
> > Nonlinear is a must IMO.  I would suggest X = exp(ln(10)/10) ~= 1.2589
> > That value has the property that a nice=10 task gets 1/10th the cpu of a
> > nice=0 task, and a nice=20 task gets 1/100 of nice=0.  I think that
> > would be fairly easy to explain to admins and users so that they can
> > know what to expect from nicing tasks.
> 
> I'm not likely to write the testcase until this upcoming weekend, though.

So that means there's a 10000:1 ratio between nice 20 and nice -19. In
that sort of dynamic range, you're likely to have non-trivial
numerical accuracy issues in integer/fixed-point math.

(Especially if your clock is jiffies-scale, which a significant number
of machines will continue to be.)

I really think if we want to have vastly different ratios, we probably
want to be looking at BATCH and RT scheduling classes instead.

-- 
Mathematics is the supreme nostalgia of our time.
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