[ntp:questions] Bug 1700 - Clock drifts excessively at pollinglevels above 256.

David L. Mills mills at udel.edu
Sun Apr 24 07:48:24 UTC 2011


Edward,

I don't know enough about the mechanism Windows uses to adjust the 
system clock. If some variant of the Unix adjtime(), the solution may be 
straightforward. The phase-lock loop parameters determine the risetime 
and overshoot of the discipline loop, in particular the loop gain and 
corer frequency. The Unix discipline loop is carefully designed for 
minimum risetime consistent with controlled overshoot of about six 
percent  The loop is designed to preserve this characteristic over a 
wide time constant an poll interval range from 8 s to 36 hr, but with 
the FLL in use at the longer time constants. The most critical parameter 
is the loop gain, which depends primarily on the timer frequency. In 
most Unix systems this is 100 Hz, but in some systems it can be as high 
as 1000 Hz with a change in parameters. It would not be feasible here to 
summarize in detail how to establish these parameters; however, Chapter 
4 of both the first and second edition of the boot "Network Time 
Synchronization: the NTP Protocol on Earth and in Space," CRC Press,, 
has the mathematical basis.

Here is a quick litmus test. With the client in normal operation with 
the pool interval at 6 (64 s) and the time and frequency settled down, 
introduce a 100-ms step in time. The discipline loop should converge to 
zero in about 3000 s and overshoot about six percent. If the response is 
far different than that, major surgery is required.

Dave,

Mischanko, Edward T wrote:

>Your question is a very good one that I don't know the answer to.  I
>have observed this behavoir while actually watching NTP Time Server
>Monitor by Meinberg, live.  I didn't anticipate any power saving
>features as being active.  I have seen a correction in the behavior by
>changing the PLL and FLL gains, as noted.  I haven't specifially looked
>for this problem in other ports, only Windows and only on systems
>without a referrence clock.
>
>-----Original Message-----
>From: questions-bounces+edward.mischanko=arcelormittal.com at lists.ntp.org
>[mailto:questions-bounces+edward.mischanko=arcelormittal.com at lists.ntp.o
>rg] On Behalf Of unruh
>Sent: Saturday, April 23, 2011 1:20 PM
>To: questions at lists.ntp.org
>Subject: Re: [ntp:questions] Bug 1700 - Clock drifts excessively at
>pollinglevels above 256.
>
>On 2011-04-22, Mischanko, Edward T <Edward.Mischanko at arcelormittal.com>
>wrote:
>  
>
>>256.
>>
>>My system clock drifts excessively when polling above 256 in a Windows
>>environment, as much as 5 ms or more.  I have made changes to
>>.../ntpd/ntp_loopfilter.c CLOCK_PLL, CLOCK_FLL, CLOCK_LIMIT, and
>>CLOCK_PGATE to address this problem.  I realize the changes I have
>>    
>>
>made
>  
>
>>are global in nature and really only need changes in the Windows port.
>>I would welcome a patch to the Windows port to accomplish these
>>    
>>
>changes
>  
>
>>or any other changes that accomplish the 1 ms stability I have now
>>achieved.  I hope Dr. Mills will have constructive comments on this
>>problem and proposed solutions.
>>    
>>
>
>Does your system cpu use powersaving cpu frequency changes? Is it a
>virtual Windows machine?
>
>  
>
>>*/
>>    
>>
>
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