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

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Message-ID: <20070716074102.GA18512@elte.hu>
Date:	Mon, 16 Jul 2007 09:41:02 +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


* Ingo Molnar <mingo@...e.hu> wrote:

> * 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.

in any case more documentation is justified, so i've added some 
clarification to the comments - see the patch below.

	Ingo

------------------------>
Subject: sched: improve weight-array comments
From: Ingo Molnar <mingo@...e.hu>

improve the comments around the wmult array (which controls the weight
of niced tasks). Clarify that to achieve a 10% difference in CPU
utilization, a weight multiplier of 1.25 has to be used.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/sched.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c
+++ linux/kernel/sched.c
@@ -736,7 +736,9 @@ static void update_curr_load(struct rq *
  *
  * 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.
+ * it's +10% CPU usage. (to achieve that we use a multiplier of 1.25.
+ * If a task goes up by ~10% and another task goes down by ~10% then
+ * the relative distance between them is ~25%.)
  */
 static const int prio_to_weight[40] = {
 /* -20 */ 88818, 71054, 56843, 45475, 36380, 29104, 23283, 18626, 14901, 11921,
-
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