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

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Message-ID: <20070418065534.GT11115@waste.org>
Date:	Wed, 18 Apr 2007 01:55:34 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	William Lee Irwin III <wli@...omorphy.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Mike Galbraith <efault@....de>,
	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 08:37:11AM +0200, Nick Piggin wrote:
> I don't know how that supports your argument for unfairness,

I never had such an argument. I like fairness.

My argument is that -you- don't have an argument for making fairness a
-requirement-.

> processes are special only because that's how we've always done
> scheduling.  I'm not precluding other groupings for fairness, though.

If you make one form of fairness a -requirement- for all acceptable
algorithms, your -are- precluding most other forms of fairness.

If you refuse to define what "fairness" means when specifying your
requirement, what's the point of requiring it?

> What do you mean optimal? If your criteria is fairness, then of course
> it is optimal. If your criteria is throughput, then it probably isn't.

I don't know what optimal behavior is. And neither do you. It may or
may not be fair. It very likely includes small deviations from fair.

> > [2] It's trivial to construct two or more perfectly reasonable and
> > desirable definitions of fairness that are mutually incompatible.
> 
> Probably not if you use common sense, and in the context of a replacement
> for the 2.6 scheduler.

Ok, trivial example. You cannot allocate equal CPU time to
processes/tasks and simultaneously allocate equal time to thread
groups. Is it common sense that a heavily-threaded app should be able
to get hugely more CPU than a well-written app? No. I don't want Joe's
stupid Java app to make my compile crawl.

On the other hand, if my heavily threaded app is, say, a voicemail
server serving 30 customers, I probably want it to get 30x the CPU of
my gzip job.

-- 
Mathematics is the supreme nostalgia of our time.
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