[ntp:questions] Re: NTP stepping issue

David L. Mills mills at udel.edu
Wed Oct 20 13:12:07 UTC 2004


Brian,

The ntpd engineered impulse response limits overshoot to about seven 
percent, with either the kernel or daemon feedback loop. What I see with 
Solaris is more like 100 percent for large initial offsets. The 
transients don't die out for many hours to a day.

I have removed questions at lists.ntp.isc.org from this reply, as that list 
complains that I am not authorized to send to it.

Dave

Brian Utterback wrote:
> David L. Mills wrote:
> 
>> Robert,
>>
>> You are not going to like this answer. I mentioned awhile back that 
>> Solaris, at least, had fiddled with the adjtime() syscall to speed up 
>> convergence for large adjustments, effectively adding an additional 
>> pole to the carefully crafted impulse response built into ntpd. The 
>> result expected is serious overshoot at large offsets, just as you 
>> describe. There is no help for it other than to use the kernel 
>> modifications, but the existing modifications were designed only for 
>> offsets less than 0.5 second.
>>
>> I don't know what system you are using, but if it slews faster than 
>> 500 PPM, expect trouble.
> 
> 
> Dave, correct me if I am wrong, but isn't a 50% overslew expected even if
> the slew rate is less than 500PPM as required? IIRC, this is one of the 
> tests
> to see if the kernel PLL is working correctly.




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