From owner-freebsd-stable@FreeBSD.ORG Fri May 11 10:11:41 2007 Return-Path: X-Original-To: freebsd-stable@FreeBSD.ORG Delivered-To: freebsd-stable@FreeBSD.ORG Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 599F616A403 for ; Fri, 11 May 2007 10:11:41 +0000 (UTC) (envelope-from olli@lurza.secnetix.de) Received: from lurza.secnetix.de (lurza.secnetix.de [83.120.8.8]) by mx1.freebsd.org (Postfix) with ESMTP id BE3E213C455 for ; Fri, 11 May 2007 10:11:40 +0000 (UTC) (envelope-from olli@lurza.secnetix.de) Received: from lurza.secnetix.de (qnunef@localhost [127.0.0.1]) by lurza.secnetix.de (8.13.4/8.13.4) with ESMTP id l4BABUFo061275; Fri, 11 May 2007 12:11:35 +0200 (CEST) (envelope-from oliver.fromme@secnetix.de) Received: (from olli@localhost) by lurza.secnetix.de (8.13.4/8.13.1/Submit) id l4BABTfh061274; Fri, 11 May 2007 12:11:29 +0200 (CEST) (envelope-from olli) Date: Fri, 11 May 2007 12:11:29 +0200 (CEST) Message-Id: <200705111011.l4BABTfh061274@lurza.secnetix.de> From: Oliver Fromme To: freebsd-stable@FreeBSD.ORG, imp@bsdimp.com, peterjeremy@optushome.com.au In-Reply-To: <20070510.225643.-713548429.imp@bsdimp.com> X-Newsgroups: list.freebsd-stable User-Agent: tin/1.8.2-20060425 ("Shillay") (UNIX) (FreeBSD/4.11-STABLE (i386)) MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-2.1.2 (lurza.secnetix.de [127.0.0.1]); Fri, 11 May 2007 12:11:36 +0200 (CEST) Cc: Subject: Re: clock problem X-BeenThere: freebsd-stable@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list Reply-To: freebsd-stable@FreeBSD.ORG, imp@bsdimp.com, peterjeremy@optushome.com.au List-Id: Production branch of FreeBSD source code List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 11 May 2007 10:11:41 -0000 M. Warner Losh wrote: > Peter Jeremy wrote: > : There seems to be a bug in ntpd where the PLL can saturate at > : +/-500ppm and will not recover. This problem seems too occur mostly > : where the reference servers have lots of jitter (ie a fairly congested > : link to them). > > Yes. This is a rather interesting misfeature of ntpd. Its rails are > at +/- 500ppm, and when it hits the rail it assumes that things are > too bad to continue and it stops. I think it is related to the maximum slew rate of 1/2000, which is equivalent to 500 ppm. The ntpd(8) manpage says: "Since the slew rate of typical Unix kernels is limited to 0.5 ms/s, each second of adjustment requires an amortization interval of 2000 s." And a bit further down: "The maximum slew rate possible is limited to 500 parts-per- million (PPM) as a consequence of the correctness principles on which the NTP protocol and algorithm design are based. As a result, the local clock can take a long time to converge to an acceptable offset, about 2,000 s for each second the clock is outside the acceptable range." > Most PC clocks have a frequency error on the order of 10-150ppm, so it > doesn't take a whole lot of jitter from a conjectsted remote network > to exceed the limits... I think the "burst" and "iburst" options for the server lines in ntp.conf might help in such cases. Of course, the best solution is to buy a GPS or DCF radio receiver and set up a startum-1 yourself. But last time I tried to do that with a cheap DCF plug, it wasn't very well supported on FreeBSD. Even an expensive Mainberg receiver ( http://www.meinberg.de/english/ ) with an RS232 output worked much more accurately with a Solaris machine than with FreeBSD. (Unfortunately, the Mainberg model availbale to us did not have NTP support via ethernet itself, only serial output.) I have to admit that that was in FreeBSD 4.x days. The situation might have improved in the meantime (I don't know). Best regards Oliver -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing b. M. Handelsregister: Registergericht Muenchen, HRA 74606, Geschäftsfuehrung: secnetix Verwaltungsgesellsch. mbH, Handelsregister: Registergericht Mün- chen, HRB 125758, Geschäftsführer: Maik Bachmann, Olaf Erb, Ralf Gebhart FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd "C++ is to C as Lung Cancer is to Lung." -- Thomas Funke