[ntp:questions] Loop Frequency and Offset

David Woolley david at ex.djwhome.demon.invalid
Tue Sep 27 09:35:19 UTC 2011

Miguel Gonçalves wrote:

> Besides correcting the CPU clock frequency what can I do to improve the

Applying a static correction to the clock frequency only helps if the 
static error is close to or greater than 500ppm.  However, if that is 
the case, the right thing to do is to replace the motherboard, with a 
better one as it is likely that the clock is unstable as well as having 
a high static error.

> performance? The machines I have at the moment are showing worse than 1 us
> performance perhaps because I am not using GPS timing receivers.

Make sure the system is only used as a time server.  Keep its 
temperature very constant.  Disable any form of power management (in 
particular variable clock rates). Disable spread spectrum clocks.

Calibrate the systematic error by outputting a hardware pulse within the 
kernel at a precisely known system clock time and using hardware to 
measure the offset from the PPS input pulse.  You will need to also 
output the system time at which you did this, but that needn't be in 
real time.

Do not try to do this on the second.

> I know... :-) My cable connection is really bad for NTP... I can't get
> better than 3 us. That is why I have 2 stratum 1 GPS receivers onsite.

Reading that as 3ms.  The time you serve will have that sort of jitter 
in, it so there is little point in being accurate to better than about 
100 microseconds of jitter/wander.

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