linux-kernel - Re: CFS review

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Message-ID: <Pine.LNX.4.64.0708011346510.1817@scrub.home>
Date:	Wed, 1 Aug 2007 14:21:21 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Mike Galbraith <efault@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

Hi,

On Wed, 1 Aug 2007, Ingo Molnar wrote:

> > [...] e.g. in this example there are three tasks that run only for 
> > about 1ms every 3ms, but they get far more time than should have 
> > gotten fairly:
> > 
> >  4544 roman     20   0  1796  520  432 S 32.1  0.4   0:21.08 lt
> >  4545 roman     20   0  1796  344  256 R 32.1  0.3   0:21.07 lt
> >  4546 roman     20   0  1796  344  256 R 31.7  0.3   0:21.07 lt
> >  4547 roman     20   0  1532  272  216 R  3.3  0.2   0:01.94 l
> 
> Mike and me have managed to reproduce similarly looking 'top' output, 
> but it takes some effort: we had to deliberately run a non-TSC 
> sched_clock(), CONFIG_HZ=100, !CONFIG_NO_HZ and !CONFIG_HIGH_RES_TIMERS.

I used my old laptop for these tests, where tsc is indeed disabled due to 
instability. Otherwise the kernel was configured with CONFIG_HZ=1000.

> in that case 'top' accounting symptoms similar to the above are not due 
> to the scheduler starvation you suspected, but due the effect of a 
> low-resolution scheduler clock and a tightly coupled timer/scheduler 
> tick to it.

Well, it magnifies the rounding problems in CFS.
I mainly wanted to test a little the behaviour of CFS and I thought a saw 
patch which enabled the use of TSC in these cases, so I didn't check 
sched_clock().

Anyway, I want to point out that this wasn't the main focus of what I 
wrote.

bye, Roman
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