Re: ntp discipline of local time?

David Woolley <david@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> writes:

Unruh wrote:
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.

I do not think that this is right, unless you are referring to a PPS
sounce. ntp sets the frequency of the kerhel clock (Is that change in
frequency what you mean by kernel time discipline) by a very simple second
order PDE feedback, and the offset by and exponential first order feedback
scheme. At least that is what it looks like to me trying to read
ntp_loopfilter.c



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.

It is the linux system I am interested in. It looks to me like it adjusts
the frequency with a simply second order feedback loop using the
ntp_adjtime system call, and then drives the
offset to zero with an exponential run once a second (?? I cannot
disentangle the code to really be sure of this) using the adjtime system
call. That exponential has a huge time constant-- something like 16 times
the poll interval.



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.

Of course. There is no way that the physical clock can be influenced by
software. The system simply changes the relation between harware cpu cycle
counts and time.


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.

I guess I was hoping that perhaps the person/people who actually wrote the
code could tell me what was going on in the code. While the code is
reasonably annotated, those annotations do not give me at least a good
sense of the overall picture.





.

Relevant Pages

  • Re: ntp discipline of local time?
    ... The Clock Discipline Principles and Precision Time Synchronization briefings on the NTP project page are old but applicable. ... factor from CPU cycles to time) and get rid of the offset. ... 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). ...
    (comp.protocols.time.ntp)
  • Re: ntp discipline of local time?
    ... Principles and Precision Time Synchronization briefings on the NTP project page are old but applicable. ... The details you are asking for are carefully explained in the NTPv4 spec. ... like the clock state machine and poll-adjust algorithm continue in the daemon. ... factor from CPU cycles to time) and get rid of the offset. ...
    (comp.protocols.time.ntp)
  • Re: ntp discipline of local time?
    ... Principles and Precision Time Synchronization briefings on the NTP ... does two things-- set the system clock frequency (ie adjust the conversion ... factor from CPU cycles to time) and get rid of the offset. ... frequency and phase errors. ...
    (comp.protocols.time.ntp)
  • Re: ntp discipline of local time?
    ... the protocol in the kernel discipline routines). ... does two things-- set the system clock frequency (ie adjust the conversion ... factor from CPU cycles to time) and get rid of the offset. ... frequency and phase errors. ...
    (comp.protocols.time.ntp)
  • Re: NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
    ... correction of offset errors is enough time to collect enough samples to ... 500ppm to prevent NTP network instabilities. ... +-100000PPM) The clock always runs forward. ... like a step from the point of the coarse sampling done by chrony or ntp, ...
    (comp.protocols.time.ntp)