[ntp:questions] Clock and Network Simulator

David L. Mills mills at udel.edu
Thu Jul 1 19:50:37 UTC 2010


Exactly as expected. The overshoot exceeds the design limit of 10 
percent by as much as 40 percent. That's exactly what the design is 
intended to avoid


Miroslav Lichvar wrote:

>On Wed, Jun 30, 2010 at 10:00:06PM +0000, David L. Mills wrote:
>>The change in SHIFT_PLL would result
>>in unstable behavior below 5 (32 s), as well as serious transients
>>if the discipline shifts from the daemon to the kernel and back. All
>>feedback loops become unstable unless the time constant is at least
>>several times the frequency update interval, which is this case is
>>one second. If you do want to explore how stability may be affected,
>>restore the original design and recompile the distribution with
>>NTP_MINPOLL changed from 3 to 1.
>I recompiled ntpd with NTP_MINPOLL 1 and here is a PLL response plot
>for SHIFT_PLL 4 poll 1 and SHIFT_PLL 2 poll 3:
>The initial offset is 0.1 second, after crossing zero offset, they
>both stay in negative.

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