[ntp:questions] is there a way to "lock" the drift frequency

wayne wayne at midwestcs.com
Fri Nov 14 20:01:56 UTC 2003



Is there a way to "lock" down the pll frequency to some value around
what is in the drift file?

I have found that on my two systems, any time the frequency wanders
too far from a known good value, that it is a sign of a problem and it
recovers *much* quicker if I simply kill the daemon and restart it
than to let ntpd work the problem out by itself.

I am using ntp 4.1.1b and 4.1.2a with Debian Linux.


The two most common causes of this are when my ADSL line gets
saturated for a long period of time (usually due to people trying to
download my entire 2GB website), or something to do with APM and lost
interrupts on my laptop.  In the case of my ADSL line being saturated,
the computer's clock is working just fine, but the huge increases in
the delays appear to cause ntp to think that it is off.  In the case
of my laptop and the dropped interrupts, the clock is off by
100-500ms, but the network is fine.  In neither case is the frequency
the cause of the problem.  In both cases, ntp will adjust the
frequency, the clock will be "corrected", but by the time the clock is
correct again, the frequency is so far off that ntp overshoots and the
frequency ends up oscillating for a long time before it settles down
again.  During this time, the offset from true time is much larger
than normal.


I have actually written up some scripts that run from cron that check
the drift file.  If frequency is off by "too much", it tries using
ntpdate to see if it can get a consistent time from the network.  If
it can, and the clock is off by "too much", it kills the server and
restarts it.  I have another quick script that goes through the
loopstats file, finds the times when the systems seems to be running
well, and finds the median frequency from those periods.  So, yes, as
the seasons change, the frequency needs to be adjusted for
temperature, but, really, the change isn't that huge.


-wayne



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