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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message
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