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

Richard B. Gilbert rgilbert88 at comcast.net
Sun Mar 30 18:38:55 UTC 2008


starlight at binnacle.cx wrote:
> Hello,
> 
> I'm trying to configure a small network for high precision time. 
> Recently acquired an Endrun CDMA time server that runs like 
> a dream, tracking CDMA time to about +/- 5 microseconds.
> 
> 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.
> 
> All are configured to prefer the Endrun clock and poll it on a 
> 16 second interval.  All are attached to a single SMC gigabit 
> Ethernet switch with only the Endrun and two Sun systems running 
> at a lower speed of 100 MBPS.  Close to zero network traffic
> and system loads.
> 
> All systems are running 'ntpd' 4.2.4p4.  Compiled NTP native 
> 64-bit for the Windows X64 system.  [A #ifdef tweak to 
> 'intptr_t' and 'uintptr_t' is required, will provide patch if 
> desired].
> 
> It generally is working well, with the systems tracking anywhere 
> from +/- 100 microseconds to +/- 500 microseconds most of the 
> time.
> 
> However once or twice a day, all the systems experience a 
> random, uncorrelated time shift of from one to several 
> milliseconds.  
<snip>

Forcing the poll interval to 16 seconds is not always a good idea!
Ntpd will select a poll interval, generally starting at 64 seconds, and 
ramping up to as long as 1024 seconds as the clock is beaten into 
submission!

Directly connected refclocks are frequently polled at shorter intervals
but I don't think your refclock is "directly connected" in the same 
sense that a clock working through a serial or parallel port is directly
connected!

A clock connected via ethernet with all the latencies and jitter 
thereunto appertaining is no different than any other network server and 
should be polled in the same manner!

The very short poll intervals correct large errors quickly and the very 
long intervals correct small errors very accurately!




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