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

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Thu Mar 15 19:42:12 UTC 2012


>
> On 2012-03-15, Ron Frazier (NTP)<timekeepingntplist at c3energy.com>  wrote:
>    
>> 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?
>>      
> Yes. The poll interval is whatever you ( or ntp sets it to). Ie, it goes
> to the externam machine to get data that often. However, ntpd only uses
> approximately 20% of the values it gets from that remote machine. Thus
> as far as the ntp algorithm on your machine is concerned its effective
> poll interval is much longer.
>
> But why in the world are you going to the NIST servers for your time?
> Use the pool. A stratum 2 or 3 server can certainly give you the ms
> accuracy you apparently want. And if you are using gps, the only purpose
> of the remote servers is a) fallback, and b) getting the second right.
> None of those require NIST.
> You can cut your polling of NIST to infinity without affecting your
> time.
>
>
>    

Allow me to explain.  A long time ago in a galaxy far far away, I wanted 
to synchronize the clocks of my Windows machines.  This was before I 
knew anything about Linux or NTP.  However, being a US citizen, I knew 
our government provided the National Institute of Standards and 
Technology (NIST) agency, and that they had a Time and Frequency 
division, just to help handle such things for US citizens and the world. 
  So, I went here to the Time and Frequency division (the link address 
may have changed over time):

http://www.nist.gov/pml/div688/

And then here to the Internet Time Service:

http://www.nist.gov/pml/div688/grp40/its.cfm

And I downloaed their little Windows time program here:

http://www.nist.gov/pml/div688/grp40/upload/nistime-32bit.exe

This thing is designed to poll the NIST servers (and only the NIST 
servers) up to every 4 hours and set your clock.  That's all it does, 
and I don't think it tries to tinker with the clock frequency.  I ran 
that for many years, and still, sometimes, would get a couple of seconds 
drift between polling intervals.  There is nothing on the website to 
discourage average Joe users from using the system.

A couple of years ago, I learned about and started using Ubuntu.  For 
timekeeping there, I used NTP.  At the time, there was a graphical 
interface to it and I picked about 8 random servers by ticking the check 
boxes and let it run.

Back around Christmas 2011, I got some atomic wall clocks and an atomic 
watch and got fascinated with the idea of getting more accuracy for the 
PC's.  That led me to doing reserach and that led me to install the 
Meinberg port of NTP.  Of course, then, I had to figure out how to add 
servers and set up ntp.conf.

In answer to your question, I'm using NIST because:

      a) My government provides it and, through their website, 
encourages the public to use it.
      b) I was familiar with it.
      c) I thought it would be more accurate.

Having said that, I could discontinue using it if the need arises.

So, I put 4 NIST servers, 4 stratum 2 servers, 4 US pool servers, and 
just for good measure, the Ubuntu time server in my ntp.conf.  Since all 
queries exiting my house show the same IP address, each computer prefers 
to use a different NIST server so one server doesn't see as many 
apparent requests.

Not long after getting NTP working on Windows, I decided to get into the 
whole GPS thing.  Hence, all the discussions I've had on this list.  My 
current goal is to set up my own GPS time server, that doesn't wander, 
and use internet servers as a backup, as you said.

>
>    
>>      
>>>> 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.
>>      
> Then there is something extremely wrong with those peers. Stop using
> them.
>
>
>    
>> 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
>>      
> AGain, that is not usual and indicates something is wrong. It is not use
> use of the internet. Something else is very wrong. Using an internet
> peer 50ms away I get jitter of less than 100us. (not ms), and I think
> that experience is far more typical. If it is your network that is the
> problem then it is especially silly to use a source like NIST, since its
> is being totally wasted.
>
>
>    

I respectfully suggest, that, if you're not in the US, then your 
experience is probably not typical of what goes on in the US.  Even in 
the country, there is very much variation in performance of the 
internet.  I cannot say whether my network, my ISP, or my country's 
internet, or all three cause the poor NTP performance.  It is what it 
is.  It's what I have to deal with.  Keep in mind that, hypothetically, 
since I'm in the Southeast, if I were to poll a server in the Northwest, 
that would be a 7000 mile round trip for the data, probably crossing 15 
or more routers.  I've given up trying to get great performance out of 
internet NTP servers.  Even with my crummy (for timekeeping) little USB 
only GPS, I'm getting 10 times better performance from it when it works 
right than I do from the internet.  I'm just going to set up my own time 
server and not worry about what the internet servers are doing.

Sincerely,

Ron



-- 

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Ron Frazier
timekeepingdude AT c3energy.com



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