Date: Fri, 23 Aug 2002 15:59:07 -0700 From: Peter Wemm <peter@wemm.org> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: John Polstra <jdp@polstra.com>, current@freebsd.org Subject: Re: how to compute the skew between TSC in SMP systems ? Message-ID: <20020823225907.970A32A7D6@canning.wemm.org> In-Reply-To: <30293.1030137693@critter.freebsd.dk>
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Poul-Henning Kamp wrote:
> In message <200208232114.g7NLE80E087984@vashon.polstra.com>, John Polstra wri
te
> s:
> >In article <29486.1030136549@critter.freebsd.dk>,
> >Poul-Henning Kamp <phk@critter.freebsd.dk> wrote:
> >> In message <20020823134410.A81962@iguana.icir.org>, Luigi Rizzo writes:
> >
> >> >does anyone have an idea on how to determine the skew between
> >> >TSC content in the various processors on an SMP box ?
> >>
> >> On i386: It cannot be reliably done. Been there, tried that.
> >
> >Well ... you can come pretty close. The BSD/OS kernel has code
> >(conditioned on option SMP_DEBUG) that manages to sync up all TSCs in
> >an N-CPU system such that the measured difference between the extremes
> >is less than 100 counts.
>
> Was this before or after APM, ACPI, SpeedStep and all the other
> crap ruined the TSC as a timecounting device ?
For what its worth, Linux does this. Remember, SMP boxes usually dont have
the same power reduction crud. And the pentium4 with the auto-throttling
has extra magic to make the TSC run at a constant speed.
Of course, this assumes the clocks are running at the same speed. I have a
machine at work that has one 800MHz cpu and one 933MHz cpu. Both are running
at 133MHz FSB but with different internal multipliers.
And then there's the machine with a 233MHz cpu and a 1.4GHz cpu... But lets
not go into that one. :-)
Cheers,
-Peter
--
Peter Wemm - peter@wemm.org; peter@FreeBSD.org; peter@yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5
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