linux-kernel - Re: CFS review

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Message-ID: <Pine.LNX.4.64.0708030041021.1817@scrub.home>
Date:	Fri, 3 Aug 2007 01:23:04 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andi Kleen <andi@...stfloor.org>, Mike Galbraith <efault@....de>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

Hi,

On Wed, 1 Aug 2007, Linus Torvalds wrote:

> So I think it would be entirely appropriate to
> 
>  - do something that *approximates* microseconds.
> 
>    Using microseconds instead of nanoseconds would likely allow us to do 
>    32-bit arithmetic in more areas, without any real overflow.

The basic problem is that one needs a number of bits (at least 16) for 
normalization, which limits the time range one can work with. This means 
that 32 bit leaves only room for 1 millisecond resolution, the remainder 
could maybe saved and reused later.
So AFAICT using micro- or nanosecond resolution doesn't make much 
computational difference.

bye, Roman
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