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

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


Uwe,

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

Guys,

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.

Dave

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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