[ntp:questions] Linux 2.6 and clock frequency

mills at udel.edu mills at udel.edu
Tue Feb 13 17:47:05 UTC 2007


shane,

As I have said before, there are two issues with Linux and clock 
frequencies other than 100 Hz. First, if the kernel discipline is used, 
the phase and frequency parameters have to be changed. Othewise, the 
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.

So, if you change to other than 100 Hz, do not use the kernel discipline 
at all. Second, be prepared for some grief using the ntpd discipline 
loop. The only other advice I can give is use FreeBSD instead.

Dave

shane-dated-1173632795.f2b9df at cm.nu wrote:
> Hey all,
> 
> I've been experimenting with Linux kernel settings with respect to how they
> affect ntpd as I understand for the gps18lvc to work effectively, one needs
> a fairly accurate clock.  One thing I've found is with hz=100, I'm getting
> an ntp.drift of around 40 but with a fairly good rootdispersion at stratum 2
> of approx 15.  With hz=250, the frequency drift drops to 15 but dispersion
> and jitter rise.  Finally, I tried the high-res timers patch with and
> without dynamic ticks.  The hr-timers have frequency at 0.039but the jitter
> reported by ntpq -p on peers is still higher than a vanilla kernel with
> hz=100.
> 
> So my question is should the drift be varying so much based on kernel hz and
> why would there be increased jitter with a reduced frequency error?  Has
> anyone else tried the hrtimers patch and had good results?
> 
> Shane
> 




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