[ntp:questions] what happens with overload on system for NTP

David Woolley david at ex.djwhome.demon.invalid
Tue Feb 9 22:04:14 UTC 2010

Ananth Shenoy wrote:
> NTP version 4.2.0


> I'am currently working on a problem on NTP. NTP log files show lot of
> drift.. The offset and frequency keep increasing and once the frequency
> rises above 500ppm the NTP displays error message.
> frequency error 513 PPM exceeds tolerance 500 PPM
> we see the offset bouncing around in +ve and -ve values sometimes high and low.
> what could be the cause of this ?

What OS.  Please confirm that you are running this on a physical machine 
as one thing that could easily cause this is trying to run nptd on a 
virtual machine.

On a real machine, this would indicate a grossly overloaded network, or 
a device driver with poor latency.  Goint to the frequency end stop in 
one direction can also mean a very bad motherboard crystal.

Power management can also cause problems, and should be disabled.
> Also, what are the effects of heavy load on the system ?
> If suppose NTP is running on low priority and some high priority

You would have a broken ntp installation.  On systems that support it, 
ntpd tries to set real time priority.

> processes continously preempt NTP from getting the CPU what will happen.
> Will then calculations of offset go wrong ?

The calculation will not go wrong, but the scheduling delays will come a 
significant part of the error term.  Note that, when working properly, 
offset is all error term.

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