[ntp:questions] NTP absolute accuracy?

Unruh unruh-spam at physics.ubc.ca
Sun Nov 1 01:20:45 UTC 2009


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).




>David




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