linux-kernel - Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers

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Date:	Fri, 10 Nov 2006 12:19:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Andrew Morton <akpm@...l.org>, tglx@...utronix.de,
	Andi Kleen <ak@...e.de>, john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Roman Zippel <zippel@...ux-m68k.org>
Subject: Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers


* Alan Cox <alan@...rguk.ukuu.org.uk> wrote:

> Ar Gwe, 2006-11-10 am 09:57 +0100, ysgrifennodd Ingo Molnar:
> > AFAIK Windows doesnt use it, so it's a continuous minefield for new 
> > hardware to break.
> 
> Windows uses it extensively especially games. The AMD desync upset a 
> lot of Windows gamers.

well, i meant the Windows kernel itself, not applications. (maybe the 
Windows kernel uses it on SMP systems where the TSC /used to be/ pretty 
stable, i dont know)

> > We should wait until CPU makers get their act together and implement 
> > a TSC variant that is /architecturally promised/ to have constant 
> > frequency (system bus frequency or whatever) and which never stops.
> 
> This will never happen for the really big boxes, light is just too 
> slow... [...]

that's not a problem - time goes as fast as light [by definition] :-)

> If hrtimer needs and requires we stop TSC support [...]

no, it doesnt, so there's no real friction here. We just observed that 
in the past 10 years no generally working TSC-based gettimeofday was 
written (and i wrote the first version of it for the Pentium, so the 
blame is on me too), and that we might be better off without it. If 
someone can pull off a working TSC-based gettimeofday() implementation 
then there's no objection from us.

	Ingo
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