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Date:      Mon, 29 Oct 2001 22:09:26 -0700
From:      Warner Losh <imp@harmony.village.org>
To:        Poul-Henning Kamp <phk@critter.freebsd.dk>
Cc:        arch@FreeBSD.ORG
Subject:   Re: 64 bit times revisited.. 
Message-ID:  <200110300509.f9U59Q701756@harmony.village.org>
In-Reply-To: Your message of "Sat, 27 Oct 2001 00:20:52 %2B0200." <613.1004134852@critter.freebsd.dk> 
References:  <613.1004134852@critter.freebsd.dk>  

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In message <613.1004134852@critter.freebsd.dk> Poul-Henning Kamp writes:
: In message <Pine.BSF.4.21.0110261622450.11653-100000@InterJet.elischer.org>, Ju
: lian Elischer writes:
: >trouble is, that ticks are:
: >1: not guaranteed to be constant
: >2/ inaccurate.
: >
: >also,
: >you can represent ticks in terms of 1/(2^64) units, certainly to the
: >accuracy of the crystals that we use for timekeeping at this time.
: 
: The 1/(10^9 * 2^32) resolution we have now would allow us to track
: the NIST Cesium fountain with about a factor of at least 50 to
: spare (we're still not sure just how good the fountain actually
: is: what do you compare the worlds best clock to ? :-)
: 
: 1/(2^64) would increase that to a safety factor of at least 185.
: 
: I have successfully been able to measure the effect of turning my
: HP5061 Cesium 90 degrees using our current code (changes the direction
: of the earths magnetic field on the cesium beam), and that is an
: effect down in the 1/10^14 range.
: 
: I'm pretty sure no crystal will give you any trouble :-)

Xtals in PCs SUCKSUCKSUCKSUCK.  They are accurate to a factor of 1E-8
or so.  This is >> 1/(2^64).  When something drifts a second or two a
day (or even a week or a month), you know it is really bad.

Warner

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