[ntp:questions] NTP offset doesn't change.

Charles Swiger cswiger at mac.com
Wed Feb 11 22:59:34 UTC 2015


On Feb 11, 2015, at 7:23 AM, Rob <nomail at example.com> wrote:
> But I see it has also been explained elsewhere in the thread: ntpd has
> a maximum on the momentary drift of 500ppm, no matter if it is static
> or dynamic or the sum of two.  I think that is not warranted.

Do you believe that a clock which loses or gains over a minute per day
should be assumed to be trustworthy?

Even a ~$10 wall clock which keeps time only by counting 50 or 60 Hz
waves from the AC line will do better than that.  While the power grid
frequency fluctuates in the short term due to load with similar magnitude of
error, the utilities make an effort to correct the time during non-peak
hours with slew rates of 200 - 333 ppm so that AC synchronous clocks
see 86400 seconds per day.

> There are also other problems with dealing with a variable drift.
> I know from observations that ntpd does not attempt to handle a
> changing drift, it only tries to lock in to the momentary drift and
> when that is changing, it keeps chasing the drift (resulting in an offset).

chrony supposedly chases the short-term offset more aggressively than
ntpd does, if that's what you prefer.

> In practice a changing drift is often caused by changing temperature,
> and it would be better to take the first derivative into account as well.

Certainly it is true that changing temperature will cause a change in crystal
frequency, on the order of ppm's to tens of ppm's per 10 C temperature change.

But if you're willing to tolerate over 500ppm systemic error, why worry about
a second-order effect in the 10s of ppm's?

Regards,
-- 
-Chuck



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