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

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Date:	Tue, 17 Apr 2007 17:08:09 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	William Lee Irwin III <wli@...omorphy.com>,
	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 11:24:22AM +0200, Ingo Molnar wrote:
> 
> * William Lee Irwin III <wli@...omorphy.com> wrote:
> 
> > [...] Also rest assured that the tone of the critique is not hostile, 
> > and wasn't meant to sound that way.
> 
> ok :) (And i guess i was too touchy - sorry about coming out swinging.)
> 
> > Also, given the general comments it appears clear that some 
> > statistical metric of deviation from the intended behavior furthermore 
> > qualified by timescale is necessary, so this appears to be headed 
> > toward a sort of performance metric as opposed to a pass/fail test 
> > anyway. However, to even measure this at all, some statement of 
> > intention is required. I'd prefer that there be a Linux-standard 
> > semantics for nice so results are more directly comparable and so that 
> > users also get similar nice behavior from the scheduler as it varies 
> > over time and possibly implementations if users should care to switch 
> > them out with some scheduler patch or other.
> 
> yeah. If you could come up with a sane definition that also translates 
> into low overhead on the algorithm side that would be great!

How's this:

If you're running two identical CPU hog tasks A and B differing only by nice
level (Anice, Bnice), the ratio cputime(A)/cputime(B) should be a
constant f(Anice - Bnice).

Other definitions make things hard to analyze and probably not
well-bounded when confronted with > 2 tasks.

I -think- this implies keeping a separate scaled CPU usage counter,
where the scaling factor is a trivial exponential function of nice
level where f(0) == 1. Then you schedule based on this scaled usage
counter rather than unscaled.

I also suspect we want to keep the exponential base small so that the
maximal difference is 10x-100x.

-- 
Mathematics is the supreme nostalgia of our time.
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