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

David L. Mills mills at udel.edu
Mon Oct 16 20:15:29 UTC 2006


I have made the point on past occasions that NTP may be more valuable as 
a hardware/software canary than as a clock winder.


Please forgive the broken "from" line in some of my messages. 
Apparently, the problem is not in the news server here, but may be in 
Netscape 7.2, which I have been using for some time and didn't use to do 
that offense. Workin' on it.


Uwe Klein wrote:
> 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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