[ntp:questions] Re: NTP stepping issue

David L. Mills mills at udel.edu
Tue Oct 19 23:27:28 UTC 2004


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

Robert Rati wrote:

> Let me test for understanding here, as I've gotten a number of replies 
> to my question.  While it is possible to configure the NTP client daemon 
> to step regardless of the time difference (as I have done), it is not 
> recommended to do so.  In my testing, I saw the client daemon slew to 
> the correct time but then it continued slewing past the correct time. Is 
> this a bug?  Once it slews to the correct time provided by the servers, 
> the client should remain synchronized, right?
> 
> Is there another way to solve the problem I laid out? (Once daemon is 
> running, always step no matter what the time difference and eventually 
> stay synchronized with the time servers)
> 
> Rob
> 
> David Woolley wrote:
> 
>> In article <416CA8D9.2050803 at udel.edu>, David L. Mills 
>> <mills at udel.edu> wrote:
>>
>>
>>> No, I did intend a small nonzero value. If I understood the question 
>>
>>
>>
>> This reply doesn't make sense.  It might have made a little more sense
>> if it weren't top posted, so I could work out which statement you were 
>> referring to, but even if we assume it is the tinker command 
>> parameters, the original article:
>>
>> 1) had no mention of any value for tinker step;
>>
>> 2) only mentioned tinkering a parameter to exactly zero;
>>
>> 3) didn't mention that you had made any prior suggestion.
>>
>> Setting step to a very small value would effectively force ntpd into
>> permanent frequency mode, stepping the time and frequency every  20 or 
>> so minutes.
>>
>>
>>> correctly, the wish was to step the clock no matter what the offset. 
>>> I wouldn't recommend that, but it can be done.
>>
>>
>>
>> The original article said that stepping was absolutely unacceptable after
>> any initialisation step.
>>
>> David Woolley wrote:
>>
>>
>>> In article <mailman.23.1097522252.72027.questions at lists.ntp.isc.org>,
>>> Robert Rati <Robert.Rati at motorola.com> wrote:
>>>
>>>
>>>
>>>> tinker panic 0
>>>
>>>
>>>
>>> I assume that this is a typo and you meant "tinker step 0".
>>
>>
>> _______________________________________________
>> questions mailing list
>> questions at lists.ntp.isc.org
>> https://lists.ntp.isc.org/mailman/listinfo/questions
>>




More information about the questions mailing list