linux-kernel - Re: Linux 3.1-rc9

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Message-ID: <20111017213143.GA8502@elte.hu>
Date:	Mon, 17 Oct 2011 23:31:43 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Simon Kirby <sim@...tway.ca>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: Linux 3.1-rc9


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

> If we used a scalar 64-bit form for all kernel internal time 
> representations:
> 
> 	s64	nsecs;
> 
> then conversions back to timespec/timeval would involve dividing 
> this 64-bit value with 1000000000 or 1000000.
> 
> Is there no faster approximation for those than bit by bit?
> 
> In particular we could try something like:
> 
> 	(high*2^32 + low)/1e9 ~==  ( high * (2^64/1e9) ) / 2^32
> 
> ... which reduces it all to a 64-bit multiplication (or two 32-bit 
> multiplications) with a known constant, at the cost of 1 nsec 
> imprecision of the result - but that's an OK approximation in my 
> opinion.

Hm, no, the numeric error would be in the *seconds* result, and would 
be 0-3 seconds - which is obviously not acceptable.

Thanks,

	Ingo
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