linux-kernel - Re: [PATCH] raise tsc clocksource rating

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Message-Id: <200710301339.23628.rusty@rustcorp.com.au>
Date:	Tue, 30 Oct 2007 13:39:23 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Glauber de Oliveira Costa <gcosta@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>, --cc@...hat.com,
	Ingo Molnar <mingo@...e.hu>, avi@...amnet.com,
	kvm-devel@...ts.sourceforge.net, John Stultz <johnstul@...ibm.com>
Subject: Re: [PATCH] raise tsc clocksource rating

On Tuesday 30 October 2007 09:17:38 Thomas Gleixner wrote:
> On Mon, 29 Oct 2007, Glauber de Oliveira Costa wrote:
>
> CC'ed John and removed glauber@....localdomain :)
>
> > From: Glauber de Oliveira Costa <glauber@....localdomain>
> >
> > tsc is very good time source (when it does not have drifts, does not
> > change it's frequency, i.e. when it works), so it should have its rating
> > raised to a value greater than, or equal 400.
> >
> > Since it's being a tendency among paravirt clocksources to use values
> > around 400, we should declare tsc as even better: So we use 500.
> >
> > This patch also touches the comments on clocksource.h, which suggests
> > that 499 would be a limit on the rating values.
> >
> > Signed-off-by: Glauber de Oliveira Costa <gcosta@...hat.com>
>
> Acked-by: Thomas Gleixner <tglx@...utronix.de>

No.  tsc is very good, it's not perfect.  If a paravirt clock registers 400 it 
really means "pick me over the tsc".

That's *why* they use > 400: it's in the documentation.

Rusty.
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