linux-kernel - Re: [PATCH] sched: fix/optimise calculation of weight-inverse

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Message-ID: <1305132185.2914.251.camel@laptop>
Date:	Wed, 11 May 2011 18:43:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Stephan Bärwolf 
	<stephan.baerwolf@...ilmenau.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nikhil Rao <ncrao@...gle.com>, Mike Galbraith <efault@....de>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix/optimise calculation of weight-inverse

On Wed, 2011-05-11 at 18:20 +0200, Ingo Molnar wrote:
> > -            lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)
> > -                / (lw->weight+1);
> > +            lw->inv_weight = WMULT_CONST / lw->weight;
> 
> hm, i definitely think there was a rounding reason for that - but apparently 
> i'm an idiot who does not add comments to non-obvious code! :-)

I suspect I might be the idiot,

> Peter, do you remember this? 

I think what we wanted to do was minimize the error:
  err = weight - inv*WMULT_CONST

by adding another term. But we could well have simply made a mess of it
instead.

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