[ntp:questions] Re: ntpd PLL and clock overshoot

Bart Van Assche bart.vanassche at gmail.com
Sun Oct 15 15:27:37 UTC 2006

A few days ago I reported that I measured a larger overshoot with nptd 
4.2.0a than with ntpd 4.1.0. By this time I have performed several test 
with ntpd 4.2.2p3, and it seems to perform significantly better than 
previous versions: overshoot is within spec and it is more accurate than 
other versions I tried (all tests have been performed with a Linux 
kernel, versions 2.4.20, 2.6.10 and 2.6.13).

Dave, can you tell me what is wrong with Linux with regard to precision 
timekeeping ?

Other people asked where the Linux implementation of the adjtimex() 
system call can be found. Its implementation resides in source file 
kernel/time.c, functions sys_adjtimex() and do_adjtimex(). See e.g. 

David L. Mills wrote:

> With all of the machines here, including FreeBSD, Solaris, HP-UX, SunOS, 
> Tru64 and HP-UX, the loop response in steady state is as I reported 
> earlier. The results with Linux are highly suspect, as at least in some 
> cases the timer interrupt frequency has been changed significantly 
> without compensation in the kernel parameters. I have recommended to 
> avoid Linux in any case involving precision timekeeping.

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