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

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Thu Mar 15 16:42:51 UTC 2012


On 3/15/2012 11:42 AM, unruh wrote:
> On 2012-03-14, Ron Frazier (NTP)<timekeepingntplist at c3energy.com>  wrote:
>    
>> On 3/14/2012 5:04 PM, Ron Frazier (NTP) wrote:
>>      
>>> On 3/14/2012 4:00 PM, David J Taylor wrote:
>>>        
>>>>> Hi David T,
>>>>>
>>>>> NOW .... you understand.
>>>>>
>>>>>            

<snip>

>> PS to my prior message.
>>
>> I don't think the problem so much is the delay to the internet servers,
>> or even to get out of my house.  NTPD is supposed to take care of that
>> as long as it's pretty much symmetrical.  I think the problem is that
>> the Windows clock is like a wild tiger that doesn't want to be tamed and
>> which is running every which way.  For whatever reason, cpu load, heat,
>> cosmic vibrations, whatever, the intrinsic frequency of the windows
>> clock is always changing.  In order to avoid beating up on the internet
>> servers too much, I have to poll them at least every 4 minutes apart.
>> If you let it, NTPD will extend that out to 16 minutes or more.  So,
>>      
> Actually, the effective NTPD poll interval is abotu 8 times the stated
> interval. The clock filter throws away about 7 out of 8 poll results in
> an attempt to get rid of assmetric polls. Ie, it assumes that the
> shortest round trip interval out of the past 8 is the best estimate of
> the symmetric roundtrip and throws away the rest. Thus if you have
> polling every 4 min  (poll interval 8) the effective interval is about
> every half hour.
>
> That is fine if the clock is an even half way reasonable clock (Ie rate does
> not change by more than say 2PPM over that time)
>
>
>    

You're saying the effective polling interval is 8x what minpoll is. 
  However, if the access policy for a NIST server is no more than 20 
times per hour or every 3 minutes, and I set minpoll to 6 or 
approximately every minute, even if the clock algorithm throws away 7 of 
8 samples; am I not still sampling?  Am I not still "hitting" the NIST 
server every minute and are they not going to ban me from accessing it 
if that continues?

>> when the clock source is polled, say the PC clock is too fast, so NTPD
>> slows it down.  Then, when you poll the clock source again, say the PC
>> clock is too slow, so NTPD speeds it up.  Because of the varying
>> intrinsic frequency of the clock, you can never find a clock speed that
>> just works, because then the system goes and changes, by changes in the
>> oscillator, how much time passes at those particular settings.  It's a
>> battle you cannot win.  By polling my GPS every 8 seconds, I can keep
>> the clock under control based on it's current needs which are varying
>> second by second.  Of course, when discussing internet servers, 30 ms of
>>      
> What are you talking about. There is no evicence either in your data or
> in any reports by anyone of 30ms variation is network offsets.
> Even on ADSL, it is in the microsecond range, not millisecond.
>
>
>    

I'm not sure exactly what you're asking.  If you're referring to my 
comment about internet peer jitter, I occasionally see jitter numbers 
for internet peers on the Meinberg Time server monitor screen in the 20 
- 30 ms range and more frequently see numbers in the 10 - 20 ms range 
for jitter.  Here is a recent screen shot:

http://dl.dropbox.com/u/9879631/internet%20jitter%20example.jpg

Note that there are three peers with jitter in the 10 - 20 ms range.

If you were asking about the offsets my computers experience using the 
internet as a time source, my TAZ computer polls the internet 
exclusively and it's offsets routinely fluctuate + / - 50 ms.

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

>> jitter doesn't help any.

<snip>


-- 

(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such.  I don't always see new messages very quickly.  If you need a
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Ron Frazier
timekeepingdude AT c3energy.com



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