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

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Fri Mar 16 04:11:39 UTC 2012


On 3/15/2012 5:06 PM, unruh wrote:
> On 2012-03-15, Ron Frazier (NTP)<timekeepingntplist at c3energy.com>  wrote:
>    
>>> 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.
>>      
> And since your early forays into timekeeping, you have learned that NIST
> does not anyone querying them more often than about 2-3 times an hour,
> that they are a stratum 1 source  which are a relatively precious
> commodity, and want to be a responsible ntp net citizen.   I am not
> blaming you for having used them, but since you have learned more, you
> have discovered more. It may be more accurate, but that accuracy is also
> useless to you, since your system does not need and cannot use
> microsecond accuracy.
>
>
>    

Actually, their policy is 20 queries / hour.  However, I do see your 
point.  I guess I'll take the NIST NTP servers out of my config file.

>
>    
>> 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
>>      
> And exactly why did you think using 13 servers would be a good thing?
> For what you are doing it is way overkill.
>
>    

It was completely arbitrary.  I was already familiar with NIST so I 
stuck them in, with each computer preferring a different server.  I 
found the stratum 2 list at ntp.org so I put them in.  I discovered the 
pool so I put them in.  And, Ubuntu has always been there since I first 
discovered NTP.

>> 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.
>>      
> That is of course great. The more people learning about time, the
> better.
>
>
>    
>>      
>>>
>>>        
>>>>
>>>>          
>>>>>> 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.
>>      
> Fine. My comments were not about myself (I certainly get far better
> accuracy out of the internet than 30ms) but from having read other
> posts here. From what I have seen most people get a lot better than 30ms
> out of internet servers.
> Before hitting on 13 different servers, it would be better tofigure out
> what the problem is that is giving you such lousy results. While I agree
> that reforming the whole internet of the Southwest USA is probably
> beyond your abilities, it may be that the problem is much closer to
> home.
>    
>

I'm sorry if that sounded offensive in any way.  I didn't mean it that 
way, and I appreciate all the advice.  Maybe I can find some time 
eventually to do some experiments with internet NTP to determine what's 
happening.

Sincerely,

Ron


-- 

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Ron Frazier
timekeepingdude AT c3energy.com



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