linux-kernel - Re: [patch v5 06/15] sched: log the cpu utilization at rq

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Message-ID: <1361373631.10155.40.camel@laptop>
Date:	Wed, 20 Feb 2013 16:20:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	torvalds@...ux-foundation.org, mingo@...hat.com,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, pjt@...gle.com,
	namhyung@...nel.org, efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org,
	morten.rasmussen@....com
Subject: Re: [patch v5 06/15] sched: log the cpu utilization at rq

On Wed, 2013-02-20 at 22:33 +0800, Alex Shi wrote:
> > There's generally a better value than 100 when using computers..
> seeing
> > how 100 is 64+32+4.
> 
> I didn't find a good example for this. and no idea of your suggestion,
> would you like to explain a bit more?

Basically what you're doing ends up being fixed point math, using 100 as
unit is inefficient, pick a power-of-2 and everything reduces to
bit-shifts.

http://en.wikipedia.org/wiki/Fixed-point_arithmetic

So use 128 or 1024 or whatever and you don't need mult and div
instructions to represent [0,1]

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