From owner-freebsd-stable Tue Jul 27 1:32:30 1999 Delivered-To: freebsd-stable@freebsd.org Received: from dingo.cdrom.com (castles554.castles.com [208.214.165.118]) by hub.freebsd.org (Postfix) with ESMTP id A9B0015378 for ; Tue, 27 Jul 1999 01:32:23 -0700 (PDT) (envelope-from mike@dingo.cdrom.com) Received: from dingo.cdrom.com (LOCALHOST [127.0.0.1]) by dingo.cdrom.com (8.9.3/8.8.8) with ESMTP id BAA01383; Tue, 27 Jul 1999 01:26:46 -0700 (PDT) (envelope-from mike@dingo.cdrom.com) Message-Id: <199907270826.BAA01383@dingo.cdrom.com> X-Mailer: exmh version 2.0.2 2/24/98 To: "David Schwartz" Cc: "Mike Smith" , stable@freebsd.org Subject: Re: Tuning the system's clock In-reply-to: Your message of "Thu, 22 Jul 1999 12:40:31 PDT." <001801bed47a$0efc31a0$021d85d1@youwant.to> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Date: Tue, 27 Jul 1999 01:26:45 -0700 From: Mike Smith Sender: owner-freebsd-stable@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG > > The TSC is calibrated at boot time using the RTC; that calibration is > > inherently limited to the accuracy of the RTC. > > Exactly. The RTC is accurate to maybe 150 ppm. The other problem is that > this calibration occurs right after power up, when the system is 'cold'. > It's not going to be representative of the system's 'warm' rate either. That assumes that you don't pause the system to allow it to warm up first. 8) > For one thing, the TSC oscillator can be replaced with a high-precision > OCXO. But that won't help you if you can't tune the TSC divisor, will it? > (Of course, XNTP would be the right tool for this purpose.) Actually, I don't think you'd have any luck replacing the TSC oscillator without a lot of nasty boardwork; many applications these days are using programmable frequency synthesisers using embedded crystals in the 10-40MHz range; I've seen others using cheap NTSC colourburst crystals and even a couple using 32kHz watch crystals. > In any event, tuning the TSC divisor via sysctl causes my FreeBSD-STABLE > machine to reboot instantly. I've just found that NTP seems to work the best > when the system is already keeping pretty good time. This is definitely a bug. Have you spoken to Poul about it yet? The "instant reboot" sounds particularly unpleasant; it should be fairly straightforward to track it down though. -- \\ The mind's the standard \\ Mike Smith \\ of the man. \\ msmith@freebsd.org \\ -- Joseph Merrick \\ msmith@cdrom.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message