linux-kernel - Re: CFS review

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Message-ID: <20070803043800.GE11115@waste.org>
Date:	Thu, 2 Aug 2007 23:38:00 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Roman Zippel <zippel@...ux-m68k.org>,
	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

On Thu, Aug 02, 2007 at 08:57:47PM -0700, Arjan van de Ven wrote:
> On Thu, 2007-08-02 at 22:04 -0500, Matt Mackall wrote:
> > On Wed, Aug 01, 2007 at 01:22:29PM +0200, Ingo Molnar wrote:
> > > 
> > > * Roman Zippel <zippel@...ux-m68k.org> 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.
> > 
> > ..which is pretty much the state of play for lots of non-x86 hardware.
> 
> question is if it's significantly worse than before. With a 100 or
> 1000Hz timer, you can't expect perfect fairness just due to the
> extremely rough measurement of time spent...

Indeed. I'm just pointing out that not having TSC, fast HZ, no-HZ
mode, or high-res timers should not be treated as an unusual
circumstance. That's a PC-centric view.

-- 
Mathematics is the supreme nostalgia of our time.
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