[ntp:questions] Setting the maximum rate of change
stenn at ntp.isc.org
Mon Mar 19 21:46:34 UTC 2007
>>> In article <45FEB162.40008 at localhost.com>, Spoon <devnull at localhost.com> writes:
Spoon> Hello everyone, Is there a run-time parameter in NTP that defines the
Spoon> maximum rate at which the clock can be adjusted (slewed?).
There is probably a "tinker" variable to do this. Be amazingly careful when
messing with these.
But I don't think you will need to - see below.
Spoon> Here's why I ask:
Spoon> I'm working with a standard that deals with 27 MHz clocks, and that
Spoon> standard states that the frequency must not change faster than 75 mHz
Spoon> per second.
If ntpd has the correct drift adjustment, you should be in good shape.
Under what exact conditions must the "don't change faster than 75mHz/sec" be
Spoon> I'll try and think aloud, in case someone can see through my
Spoon> Consider H : a 26,999,900 Hz clock.
Spoon> Thus H has an offset of 100 Hz = 3.7 ppm
Spoon> In other words, H "misses" 100 ticks every second.
If left uncorrected, yes.
Spoon> I'd have to add 100 / 27e6 = 3.7 µs every second to keep H from
Spoon> drifting away from the correct time.
Once ntpd has achieved "state 4", ntpd will calculate the drift and will
automatically handle this.
Spoon> But I can't make that change all of a sudden, I can only change 0.075
Spoon> Hz more (2.78 ns) every second.
ntpd will apply the correction on a per-tick basis, not all at once, once a
Are you comfortable that the specs of the machines you are using will keep
time well enough that their clocks can be kept in-sync by limiting
adjustment to 75mHz/sec?
Spoon> Would I improve precision if I changed HZ from 100 to 250?
Exactly what do you mean by "precision"?
Have you seen http://ntp.isc.org/Support/NTPRelatedDefinitions ?
And have you read about the problems you can cause by having HZ at a value
other than 100? Particularly with Linux kernels?
More information about the questions