[ntp:questions] ntpd -x option

David Woolley david at ex.djwhome.demon.co.uk.invalid
Fri Mar 13 07:56:25 UTC 2009

Matthias Fuchs wrote:

> But now, when I start ntpd with the -x option and an offset of
> about 30s before ntpd is started, I get error messages like this
>         frequency error 557 PPM exceeds tolerance 500 PPM

I should also have pointed out that the 500ppm slew rate limit is 
imposed by *n*x kernels, not by ntpd.  In the case of -x, which forced 
the user space discipline, slews are implemented by periodically telling 
the kernel to slew by a certain amount of time (not a rate).  The kernel 
does this by changing the amount it adds to the high resolution software 
clock for every tick until that correction is complete.  The amount of 
the change is generally fixed at +/-500ppm (although it is less on some 
older systems).

This worked where the clock could only be read to a resolution of one 
tick. However, I would want to check the kernel sourced code before I 
would be prepared to say that this produces strictly monotonic time with 
the tick interpolation schemes used on modern systems.

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