[ntp:questions] NTPD can take 10 hours to achieve stability

unruh unruh at wormhole.physics.ubc.ca
Mon Apr 18 06:31:04 UTC 2011

On 2011-04-18, David J Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> "C BlacK" <rblak at non.net> wrote in message 
> news:o-adnVWgbZB3NTbQnZ2dnUVZ_oCdnZ2d at supernews.com...
>> Why would it take ntpd ten hours to achieve its accuracy?  Can this be 
>> explained in laymans terms and
>> mathematically
> What accuracy do /you/ want?  For best accuracy which NTP can achieve, in 

He was refering to the statement that ntpd can take up to 10 hr to
achieve its accuracy. That is the time taken for ntpd to settle down to
an accuracy of a few usec if they start off with a few 10s of PPM in
rate error. 

> the order of microseconds or better, your PC may need to be in a 
> temperature controlled environment - how long would it then take to 

No. With a GPS clock, ntp will run with a few usec error ( about
3-5usec) even in a non-temp corrected environment. But it is clear
looking at the errors that it is because ntpd takes so long to settle
down that is dominating the errors. 

> stabilise its own temperature?  Compare people using the best frequency 
> sources, who may need to leave them for days to achieve their best 
> accuracy.  On the other hand, if all you need is, say, 20ms (the clock 
> interrupt period on many Windows PCs, so as accurate as a typical program 
> could measure) then NTP might be that accurate just as soon as you've 
> logged in.

Yup. that is certainly true. If all you want is a few ms, then ntp will
settle down in a few hours. (See the graph at the bottom of 
www.theory.physics.ubc.ca/chrony/chrony.html where ntpd started off with
with a rate error of 40PPM and took a few hours to settle down to a few
ms accuracy.)

> The time taken to stabilise will typically depend on what accuracy /you/ 
> want to achieve, so "ten hours" alone is meaningless out of context.
> Cheers,
> David 

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