From owner-freebsd-arch  Wed May 16 19:56: 2 2001
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To: Nik Clayton <nik@FreeBSD.ORG>
Cc: Matt Dillon <dillon@earth.backplane.com>,
	Bruce Evans <bde@zeta.org.au>,
	Terry Lambert <tlambert2@mindspring.com>,
	dave <dleimbac@earthlink.net>, freebsd-questions@FreeBSD.ORG,
	arch@FreeBSD.ORG
Subject: Re: Gettimeofday Again... 
In-Reply-To: <20010517012753.A99822@catkin.nothing-going-on.org> 
Date: Wed, 16 May 2001 19:55:48 -0700
From: Peter Wemm <peter@wemm.org>
Message-Id: <20010517025548.35895380E@overcee.netplex.com.au>
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Nik Clayton wrote:

> On Wed, May 16, 2001 at 10:43:14AM -0700, Matt Dillon wrote:
> >     I don't change the timercounter method defaults, and I sure hope you
> >     aren't advocating that people change their timecounter defaults.  If
> >     the TSC is a reasonable default, the system should figure it out and
> >     use it without requiring intervention.
> 
> At the risk of going off at a slight tangent, TSC is not a good default
> on some hardware.  In particular, at least some laptops will start to
> gain or lose time almost immediately they boot if the timecounter is set
> to TSC.  It has to be set to i8254 in /etc/sysctl.conf.  This is covered
> in the FAQ.

FYI: Pentium4 cpus have:
Features=0x3febfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,
  MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,<b28>,ACC>

The last one is interesting. As I understand it, ACC ("Auto Clock
Correction") allows for TSC correction in spite of varying cpu clocks.

This is important because of the variable cpu speed throttling to keep the
heat down.

Cheers,
-Peter
--
Peter Wemm - peter@FreeBSD.org; peter@yahoo-inc.com; peter@netplex.com.au
"All of this is for nothing if we don't go to the stars" - JMS/B5


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