[ntp:questions] NTP absolute accuracy?

David Lord snews at lordynet.org
Sun Nov 1 03:10:51 UTC 2009


Unruh wrote:
> David Lord <snews at lordynet.org> writes:
> 
>> David J Taylor wrote:
>>> "Unruh" <unruh-spam at physics.ubc.ca> wrote in message 
>>> news:qg_Gm.50009$Db2.46585 at edtnps83...
>>>> "David J Taylor" 
>>>> <david-taylor at blueyonder.not-this-bit.nor-this.co.uk.invalid> writes:
>>>>
>>>>
>>>>> "David J Taylor"
>>>>> <david-taylor at blueyonder.not-this-bit.nor-this.co.uk.invalid> wrote in
>>>>> message news:%XQGm.126$Ym4.58 at text.news.virginmedia.com...
>>>>>> "Unruh" <> wrote in message news:34JGm.50898$PH1.36449 at edtnps82...
>>>>>> []
>>>>>>> Note that chrony will give you a factor of 2 or three improvement over
>>>>>>> ntp  in the errors, assuming that the roundtrip is equally split on
>>>>>>> Linux or BSD.
>>>>>> For those without wide-bandwidth academic connections - those folks on
>>>>>> cable or ADSL - how good is an "equal split round trip" assumption?
>>>>>>
>>>>>> David
>>>>> Thanks, David, David and Jan.  A few milliseconds is what I had 
>>>>> expected,
>>>>> so if you are on a consumer line, what implications does that have for
>>>>> unruh's comment?
>>>>> Cheers,
>>>>> David
>>>> Which comment?
>>> The one I quoted:
>>>
>>> "Note that chrony will give you a factor of 2 or three improvement over 
>>> ntp  in the errors, assuming that the roundtrip is equally split on 
>>> Linux or BSD."
>>>
>>> On consumer-grade circuits the assumption about equal round trip is 
>>> unlikely to be valid.
>>>
>>> I hope that David Lord can make some tests, and I await the results with 
>>> interest.
> 
>> Both fileserver using ntpd and desktop using chrony had only my
>> local ntp servers as sources so I've restarted both with three
>> remote sources, one common source at my isp, and alternate
>> sources from two different sites.
> 
>> What I'm most interested in is the effect of temperature as it's
>> one of two periods in year that gets largest temperature
>> variations and I've already had to restart the servers as ntpd
>> was unable to keep up with the higher minpoll values I had set.
> 
>> I'm now not quite sure how best to compare stats from chrony vs
>> ntpd.
> 
> Well, the way I did it was to attach a GPS PPS to the computer and use the offset
> between it and the system time as a measurement of the accuracy. 
> (the gps was NOT used as a source for eitehr ntp or chrony).
> 

How do you get the time difference between your GPS and system
time?

At the moment there only seems to be 100us between offsets and
apart from this being insignificant I wouldn't know which was
nearer to ntp time.

My adsl connection has also been off already for 30 minutes
after I'd set this up. One of my servers that gets time from
MSF and also has both test systems as sources. Both test
systems also have the common source of my isp's timeserver
so I'm expecting to get something meaningful from that.

I might be able to get GPS to both systems but not for a while.

David




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