[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.

Symtoms:
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.

uwe




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