[ntp:questions] ntp discipline of local time?
david at ex.djwhome.demon.co.uk.invalid
Tue Mar 25 13:25:48 UTC 2008
> How does ntp actually discipline the local clock? I have a gps received
If you are using the kernel time discipline, which you should be using
for high accuracy, nptd doesn't discipline the clock; it is the kernel
code that does that, based on measurements provided by ntpd.
> attached to a computer which is disciplined by a remote clock over an ADSL
> line. (Ie, the gps does not act as a refclock -- it is purely to measure
> the actual offset of the system. It is only the remote server that actaully
> acts the ntp reference source.)
> I can watch how ntp alters the local clock in response to remote
> offsets. The response is not linear. rather it is "curved" as though the
> rate of the local clock were exponentially eliminating the offset. But this
That sounds very plausible. The clock discipline code solves for both
frequency and phase errors. The phase error is probably being filtered
using an IIR filter, and that is what you are seeing, and also the
mechanism ntpd uses to stop wandering off if it stops receiving updates
(the frequency measurement error can produce unbounded phase errors, but
the phase error correction is bounded).
> is between two succesive runnings of the loopstats. Where is this behaviour
> determined? -- ie which routines determines the response of the system
> between to successive measurements of the offset?
If you don't use the kernel discipline, on Unix-like systems, it will
implement the same filters in user space and apply phase adjustments at
each kernel update. For ntpv3, those updates were every 4 seconds; for
ntpv4, I believe it does them every second. A normal Unix-like system
will implement the phase change by increasing or decreasing the amount
by which the software clock is updated for every tick by +/- 500ppm,
until the adjustment is complete.
Windows has a different kernel interface, and I believe that ntpd
modulates the effective length of a tick.
Note, in spite of what other replies may imply, the physical clock
frequency is never actually changed; what is actually changed is the
amount by which the software clock is incremented for ever n-cycles of
whatever is used for the reference frequency.
If you want the actual code and fine details, you will be able to find
them as easily as I will, so I'll leave that as an exercise for the reader.
More information about the questions