Re: ntp discipline of local time?

Unruh,

The kernel discipline is almost identical to the daemon discipline with the exception that the fancy code to combine the PLL and FLL near the Allan intercept is absent. Without the PPS signal, the discipline behaves as a second-order loop; with the PPS it behaves as two separate first-order loops, one for phase, the other for frequency. When necessary to set the freque directly, ther kernel is use for that. Note that the peripheral functions, like the clock state machine and poll-adjust algorithm continue in the daemon.

Dave

Unruh wrote:
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?
    ... Without the PPS signal, the discipline ... 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 discipline of local time?
    ... 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. ... 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)
  • Time Drift Compensation on Linux Clusters
    ... While working on a Linux cluster with kernel version 2.4.27 we've ... for this problem which is based on the Pentium's TSC clock. ... Following is a detailed description of the problem and the fix, ... The number of interrupt per second is defined by ...
    (Linux-Kernel)