linux-kernel - Re: [patch] CFS scheduler, -v18

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Date:	Tue, 3 Jul 2007 09:22:52 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Galbraith <efault@....de>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Keith Packard <keith.packard@...el.com>
Subject: Re: [patch] CFS scheduler, -v18

* Mike Galbraith <efault@....de> wrote:

> This doesn't appear to be a CFS problem.  I can reproduce the problem 
> easily in virgin 2.6.22-rc7 by starting xterm-spam at nice -1 or 
> better. As soon as xterm-spam can get enough CPU to keep the xterm 
> fully busy, it's game over, the xterm freezes.  The more accurate 
> fairness of CFS to sleepers just tips the balance quicker.  In 
> mainline, the xterm has an unfair advantage and maintains it 
> indefinitely... until you tip the scales just a wee bit, at which time 
> it inverts.

ah. That indeed makes sense. It seems like the xterm doesnt process the 
Ctrl-C/Z keypresses _at all_ when it is 'spammed' with output. Normally, 
output 'spam' is throttled by the scroll buffer's overhead. But in 
Vegard's case, the printout involves a \r carriage return:

       printf("%ld\r", 1000 * clock() / CLOCKS_PER_SEC);

which allows xterm-spam (attached) to easily flood the xterm (without 
any scrolling that would act as a throttle) and the xterm to flood Xorg. 

I suspect we need the help of an xterm/Xorg expert? (maybe Keith can 
give us further pointers? I can reproduce the problem on a T60 with i940 
and Core2Duo running Fedora 7 + Xorg 7.1.)

	Ingo

View attachment "xterm-spam.c" of type "text/plain" (102 bytes)