Date: Wed, 17 Jul 2002 23:01:46 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: dmp@pantherdragon.org Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: Counting the clock cycles Message-ID: <20020717.230146.85933196.imp@bsdimp.com> In-Reply-To: <3D3648A2.DADFE507@pantherdragon.org> References: <3D35D2D0.F480C81D@pantherdragon.org> <20020717.221713.18991134.imp@bsdimp.com> <3D3648A2.DADFE507@pantherdragon.org>
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In message: <3D3648A2.DADFE507@pantherdragon.org>
Darren Pilgrim <dmp@pantherdragon.org> writes:
: "M. Warner Losh" wrote:
: >
: > In message: <3D35D2D0.F480C81D@pantherdragon.org>
: > Darren Pilgrim <dmp@pantherdragon.org> writes:
: > : If you're really worried about it, get a GPS device that can provide
: > : you with a PPS signal for use with ntpd. Then I'd say you could safely
: > : rely on the computer's clock being accurate.
: >
: > If you are lucky enough to find "accuracy" in the 10s of us close enough.
:
: I don't quite understand what you're saying here.
I'm saying that ntp steers the system clock only +- 40-70us (eg a few
10's of us) in the best client situation on a LAN. Reference clocks
can get the system time deviation down to a few microseconds (on the
order of 2-5us). phk claims to have gotten better with custom clock
hardware... of course these numbers are from FreeBSD 4.3 and the ntp
nano-kernel in the kernel has gotten a little better since then.
Warner
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