Date: Fri, 11 May 2007 12:11:29 +0200 (CEST) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-stable@FreeBSD.ORG, imp@bsdimp.com, peterjeremy@optushome.com.au Subject: Re: clock problem Message-ID: <200705111011.l4BABTfh061274@lurza.secnetix.de> In-Reply-To: <20070510.225643.-713548429.imp@bsdimp.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200705111011.l4BABTfh061274>