[ntp:questions] Linux 2.6 and clock frequency

David Woolley david at djwhome.demon.co.uk
Thu Feb 15 07:10:44 UTC 2007

In article <eqstit$gjm$1 at scrotar.nss.udel.edu>, mills at udel.edu wrote:

> feedback loop dynamic response will go nuts. Second, the slew rate of 
> the Unix tickadj() system call needs to be changed to preserve the 
> assumed slew rate of 500 PPM. The Linux kernelmongers probably know this 
> but so far have not made the necessary changes.

The last time that I looked at the code, this was done, but, like
ntpq, subject to a "precision" of only one microsecond.  That means
that the slew rate will be 500ppm for clock periods that are a multiple
of 2ms, i.e. 100 Hz, 125Hz, 166.666666.... Hz, 250Hz, and 500Hz.  Of
these, apart from 100Hz, only 250Hz is sometimes used, but 1000Hz is
rather more common, and doesn't have an integral number of microseconds
per tick that will achieve 500ppm.  (I'm not sure if the repeating 
decimal frequency can actually be configured acurately, though.)

> So, if you change to other than 100 Hz, do not use the kernel discipline 

This is referring to the first of Dave Mills' comments.

> at all. Second, be prepared for some grief using the ntpd discipline 
> loop. 

This is referring to the slew rate issue.

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