[ntp:questions] high precision tracking: trying to understand sudden jumps

David Woolley david at ex.djwhome.demon.co.uk.invalid
Sun Mar 30 19:10:21 UTC 2008

starlight at binnacle.cx wrote:

> The clients are a rag-tag assembly of diverse systems including 
> a Centos 4.5 Linux i686, Linux x86_64, Sun Ultra 10, Sun Ultra 80, 
> IBM RS/6000 44p, Windows 2003 X64, and a Windows XP laptop.

How are you interpolating the 16ms ticks on the Windows system?  How are 
you disabling power management on the lap top?

> It generally is working well, with the systems tracking anywhere 
> from +/- 100 microseconds to +/- 500 microseconds most of the 
> time.

How are you measuring the difference from true time?  In principle, if 
ntpd can measure it, it will correct it.

> However once or twice a day, all the systems experience a 
> random, uncorrelated time shift of from one to several 
> milliseconds.  Had an issue where a UPS voltage correction shift 

In which direction is the slip?  Backward only slips against true time 
(these might appear as forward slips if the real error is in the server) 
are typically due to lost clock interrupts.  If that is the case it 
implies you are using a tick rate of other than 100Hz.  Please note that 
the Linux kernel code is broken for clock frequencies other than 100Hz 
and the use of 1000Hz significantly increases the likelihood of a lost 

The normal source of lsot interrupts is disk drivers using programmed 

> and cheap power supply on the Windows X64 box appeared to be a
> problem, but that was fixed by configuring the UPS to consider 
> 110V nominal instead of 120V.

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