From owner-freebsd-hackers Wed Jul 17 22: 2:56 2002 Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.FreeBSD.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 13A1F37B401 for ; Wed, 17 Jul 2002 22:02:54 -0700 (PDT) Received: from harmony.village.org (rover.bsdimp.com [204.144.255.66]) by mx1.FreeBSD.org (Postfix) with ESMTP id 513BA43E3B for ; Wed, 17 Jul 2002 22:02:53 -0700 (PDT) (envelope-from imp@bsdimp.com) Received: from localhost (warner@rover2.village.org [10.0.0.1]) by harmony.village.org (8.12.3/8.12.3) with ESMTP id g6I52p1f022979; Wed, 17 Jul 2002 23:02:51 -0600 (MDT) (envelope-from imp@bsdimp.com) Date: Wed, 17 Jul 2002 23:01:46 -0600 (MDT) Message-Id: <20020717.230146.85933196.imp@bsdimp.com> To: dmp@pantherdragon.org Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: Counting the clock cycles From: "M. Warner Losh" In-Reply-To: <3D3648A2.DADFE507@pantherdragon.org> References: <3D35D2D0.F480C81D@pantherdragon.org> <20020717.221713.18991134.imp@bsdimp.com> <3D3648A2.DADFE507@pantherdragon.org> X-Mailer: Mew version 2.1 on Emacs 21.1 / Mule 5.0 (SAKAKI) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.ORG In message: <3D3648A2.DADFE507@pantherdragon.org> Darren Pilgrim writes: : "M. Warner Losh" wrote: : > : > In message: <3D35D2D0.F480C81D@pantherdragon.org> : > Darren Pilgrim 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 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message