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

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Message-Id: <20070418221432.e4dbcf4f.akpm@linux-foundation.org>
Date:	Wed, 18 Apr 2007 22:14:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	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,
	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 Thu, 19 Apr 2007 05:18:07 +0200 Nick Piggin <npiggin@...e.de> wrote:

> And yes, by fairly, I mean fairly among all threads as a base resource
> class, because that's what Linux has always done

Yes, there are potential compatibility problems.  Example: a machine with
100 busy httpd processes and suddenly a big gzip starts up from console or
cron.

Under current kernels, that gzip will take ages and the httpds will take a
1% slowdown, which may well be exactly the behaviour which is desired.

If we were to schedule by UID then the gzip suddenly gets 50% of the CPU
and those httpd's all take a 50% hit, which could be quite serious.

That's simple to fix via nicing, but people have to know to do that, and
there will be a transition period where some disruption is possible.

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