Re: clock problem
- From: Oliver Fromme <olli@xxxxxxxxxxxxxxxxx>
- Date: Sat, 12 May 2007 13:08:24 +0200 (CEST)
M. Warner Losh wrote:
Oliver Fromme wrote:
: 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."
I think you are confusing two things here. One is the maximum
frequency error of the system clock that ntpd can tolerate. The other
is the maximum slew rate of the system clock.
I'm aware of that. It just caught my eye that both of
those values are the same.
: 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).
My company has used FreeBSD's ntpd since 3.x with a small, custom
driver that I wrote. It turns out to work very well in practice. I'd
suggest that it is well supported, even in FreeBSD 4.x. It isn't well
documented.
I guess you're talking about the kernel part. That's
probably true. However, we had two problems (with that
Meinberg receiver in particular):
First, there was no working driver. I had to hack one
of the existing drivers so it would work with the
signals sent from the Meinberg clock. And I agree that
it isn't well documented.
Second, the serial port code of FreeBSD was a problem.
It caused terrible jitter, and I wasn't able to fix it.
It was completely unusable. I'm afraid I don't have
exact numbers anymore, it has been a few years ago, but
when we connected the same receiver to a Solaris machine,
the jitter went away.
On a related note, some time ago I got a cheap DCF radio
receiver for my private use (~ EUR 50). It has a parallel
port connector, I think. I was unable to find a driver
that would work under FreeBSD. It caused me to do a bit
of research about what receivers are actually supported
under FreeBSD, and which can be bought without too much
trouble (without paying EUR 1000 like those Meinberg boxes
cost). I couldn't find any. The ntp docs mention some
devices that seemingly could be bought 10 years ago, but
they're not available anymore today.
Well, so now I'm back to using a bunch of public NTP
servers in my ntp.conf, and the DCF receivers catches
dust in some cardboard box in the attic. :-)
Best regards
Oliver
--
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FreeBSD-Dienstleistungen, -Produkte und mehr: http://www.secnetix.de/bsd
"I have stopped reading Stephen King novels.
Now I just read C code instead."
-- Richard A. O'Keefe
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- From: M. Warner Losh
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