[ntp:questions] Is rate of slew adjustment more or less determinate?

J. Jobin jobin at hotmail.com
Tue Sep 15 22:48:34 UTC 2009


According to this page:

http://doc.ntp.org/4.2.2p4/ntpd.html
If one were to use start ntpd with -x,
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-x 
Normally, the time is slewed if the offset is less than the step threshold, which is 128 ms by default, and stepped if above the threshold. This option sets the threshold to 600 s, which is well within the accuracy window to set the clock manually. Note: Since the slew rate of typical Unix kernels is limited to 0.5 ms/s, each second of adjustment requires an amortization interval of 2000 s. Thus, an adjustment as much as 600 s will take almost 14 days to complete. This option can be used with the -g and -q options. See the tinker command for other options. Note: The kernel time discipline is disabled with this option.

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Is this more or less true, i.e., the correction can take up to 2000 seconds for every one second? Is there any way one could speed it up without resorting to step adjustment? I would prefer not to use the tinker command because of all the caveats mentioned about its use.
Also, is it possible to find the current slew rate? I'm using Linux kernels 2.6.19 and 2.6.24.
The reason I'm asking is I have a program which tries to use slew adjustment to correct time. I need to know how long the adjustment will take and I was wondering if I could simply take the current time difference and multiply it by 2000 for every second in difference (and perhaps add an extra buffer amount) in order to get an idea. 
Thanks,
jobin

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