[ntp:questions] ntp discipline of local time?

Unruh unruh-spam at physics.ubc.ca
Wed Mar 26 00:28:23 UTC 2008


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

>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       */

The program interacts with the kernel through ntp_adjtime ( adjtimex on
linux) I believe, so lets keep the discussion at that level.

 From what I see in ntp_loopfilter.c in 




>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:

NO, it also feeds it a frequency offset. 
I am not objecting to a simple exponential decay being an IIR filter. It is
a very specific IIR filter. 



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

>    172  #define MOD_BITS (MOD_OFFSET | MOD_MAXERROR | MOD_ESTERROR | \
>    173      MOD_STATUS | MOD_TIMECONST)

>gives the default set of ntpadjtime mode options.

And then line 595-597
           ntv.modes |= MOD_FREQUENCY;
            ntv.freq = (int32)((clock_frequency +
                drift_comp) * 65536e6);


 And then the whole of 
/*
 * adj_host_clock - Called once every second to update the local clock.
 *
 * LOCKCLOCK: The only thing this routine does is increment the
 * sys_rootdispersion variable.
 */

whose key line is
   adjustment = clock_offset / (CLOCK_PLL * min(ULOGTOD(sys_poll),
       allan_xpt));
   clock_offset -= adjustment;
   adj_systime(adjustment + drift_comp);

Now it is not clear to me that this is actually run once a second, or if
this is only run once a second under certain conditions, or what.

This certainly produces an exponential decay. 






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