[ntp:questions] Setting the maximum rate of change

Harlan Stenn 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> confusion.

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 mailing list