[ntp:questions] ntp discipline of local time?

David Woolley david at ex.djwhome.demon.co.uk.invalid
Tue Mar 25 21:52:46 UTC 2008

Unruh wrote:
> David Woolley <david at ex.djwhome.demon.co.uk.invalid> writes:
>> 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

2.4 kernel (2.4.33): /usr/src/linux/kernel/timer.c

    417      /*
    418       * Compute the phase adjustment for the next second. In
    419       * PLL mode, the offset is reduced by a fixed factor
    420       * times the time constant. In FLL mode the offset is
    421       * used directly. In either mode, the maximum phase
    422       * adjustment for each second is clamped so as to spread
    423       * the adjustment over not more than the number of
    424       * seconds between updates.
    425       */

loopfilter.c feeds the kernel with offset when it has a time update and 
it is in PLL mode.  Note that a simple exponential decay is an IIR filter:

 From ntp v4.2p4: ntpd/ntp_loopfilter.c


gives the default set of ntpadjtime mode options.

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