[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Wed Mar 14 15:41:00 UTC 2012


On 3/14/2012 10:41 AM, David J Taylor wrote:
>> Hi David,
>>
>> OK.  You asked for it.  8-)
>
> Well, I actually suggested /all/ the Internet servers being enabled, 
> allowing NTP to make its best choice.
>
>> When I'm through testing, I'll open up the other internet servers as 
>> a backup in case the GPS fails.  For now, I'm just running with one 
>> clock source at a time.  Still trying to document and chase down this 
>> wandering effect.
>>
>> I ran with NY NIST as the only selectable clock source and monitoring 
>> the GPS for comparison all night.  The results were horrible.  My 
>> offsets from NIST time were in the + 65 ms / - 75 ms range.  I had 
>> the polling interval set to start at 1 minute and go up to 4 
>> minutes.  There is way too much clock wander to even think about 
>> testing the accuracy of the GPS.  I've gone back to polling the GPS 
>> every 8 seconds as the sole selectable clock source and monitoring 
>> the internet servers for comparison.  Over the short term, minutes to 
>> hours, my GPS, even with NMEA only, is by far the most accurate time 
>> source I have.  Even if the NMEA signal wanders 70 ms either way over 
>> the course of a few days, it won't get any further off than I did 
>> using the internet server, and the clock will be much more consistent 
>> over shorter time frames.
>>
>> Here are the graphs.
>>
>> http://dl.dropbox.com/u/9879631/nynist01.jpg
>> http://dl.dropbox.com/u/9879631/nynist02.jpg
>>
>> Sincerely,
>>
>> Ron
>
> I'm not surprised that using a single Internet server is worse than 
> the GPS/USB, but that's not how NTP is designed to work.  With two or 
> more Internet servers active, your GPS 50-second glitch would not have 
> affected your PC's timekeeping anything like as severely, when you 
> have the GPS/USB included to help improve the offset and more like the 
> narrow band (about 15 milliseconds wide) shown in:
>
>  http://dl.dropbox.com/u/9879631/drifting01-peerstats.20120312.jpg
>
> Cheers,
> David
>

OK.  Here are the loopstats from another computer for 7 days (in the 
chart).  I don't have any peerstats for it.  It has the same server 
list.  One is preferred.  All servers are active.  Performance is no better.

http://dl.dropbox.com/u/9879631/TAZ%20loopstats%202012-03-07%20to%202012-03-14.jpg
http://dl.dropbox.com/u/9879631/ntp.conf-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120313-TAZ
http://dl.dropbox.com/u/9879631/loopstats.20120314-TAZ

Sincerely,

Ron


-- 

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Ron Frazier
timekeepingdude AT c3energy.com



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