linux-kernel - Re: [PATCH] CFS: Fix missing digit off in wmult table

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Message-ID: <20070716070610.GA10907@elte.hu>
Date:	Mon, 16 Jul 2007 09:06:10 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	James Bruce <bruce@...rew.cmu.edu>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Mike Galbraith <efault@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH] CFS: Fix missing digit off in wmult table


* James Bruce <bruce@...rew.cmu.edu> wrote:

> While we're at it, isn't the comment above the wmult table incorrect? 
> The multiplier is 1.25, meaning a 25% change per nice level, not 10%.

yes, the weight multiplier 1.25, but the actual difference in CPU 
utilization, when running two CPU intense tasks, is ~10%:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 8246 mingo     20   0  1576  244  196 R   55  0.0   0:11.96 loop
 8247 mingo     21   1  1576  244  196 R   45  0.0   0:10.52 loop

so the first task 'wins' +10% CPU utilization (relative to the 50% it 
had before), the second task 'loses' -10% CPU utilization (relative to 
the 50% it had before).

so what the comment says is true:

 * The "10% effect" is relative and cumulative: from _any_ nice level,
 * if you go up 1 level, it's -10% CPU usage, if you go down 1 level
 * it's +10% CPU usage.

for there to be a ~+10% change in CPU utilization for a task that races 
against another CPU-intense task there needs to be a ~25% change in the 
weight.

	Ingo
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