[ntp:questions] Re: ntpd PLL and clock overshoot

Uwe Klein uwe_klein_habertwedt at t-online.de
Sun Oct 15 15:59:55 UTC 2006

David L. Mills wrote:

> With all of the machines here, including FreeBSD, Solaris, HP-UX, SunOS, 
> Tru64 and HP-UX, the loop response in steady state is as I reported 
> earlier. The results with Linux are highly suspect, as at least in some 
> cases the timer interrupt frequency has been changed significantly 
> without compensation in the kernel parameters. I have recommended to 
> avoid Linux in any case involving precision timekeeping.

Hello Dave,
there is at least one issue with APIC routed interupts on linux running
on nVidia nForce 1 and 2 based Boards resulting in a too fast and irregular clock.

It seems the timer interrupt is handled _twice_ on occasion.

I have A7N266-E and A7N8X-E boards produce this problem with various kernels
in the 2.6.1n range.

The same boards ran ntpd on linux 2.4 and no APIC routing just perfect.

The clock starts to run ahead by ~8-900ppm resulting in hard correction
of -.5 to -1.5 seconds every couple of hours. Adjusting the system tick value
results in symetric corrections +.5 .. -.5  which would indicate an extremely
unstable clock.

This started out for me as a problem with ntpd not syncing
BUT is now Linux/Hardware related with ntpd being the whistle-blower.

One of the reasons i started reading this group some weeks ago.


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