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

David J Taylor david-taylor at blueyonder.co.uk.invalid
Mon Apr 18 10:16:53 UTC 2011


"unruh" <unruh at wormhole.physics.ubc.ca> wrote in message 
news:slrniqnml8.ln1.unruh at wormhole.physics.ubc.ca...
> 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.

So, there is an implicit accuracy related to that 10 hour statement, which 
may not apply to someone else's needs.

>> 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.

Mine is within about 6 microseconds, in a non-temperature controlled 
environment (running 24 x 7).  Looking at the plots it is quite clear that 
temperature variations are the limiting factor in accuracy, and not NTP.

>> 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.)

Then it could be that you may not not seeing the best which NTP can 
achieve, if you need to wait "a few hours" rather than a few minutes.  The 
systems running here, those which are not running 24 x 7, are typically 
already within few ms after booting up, when you log in.  Not always, but 
very often.  For example:

  http://www.satsignal.eu/mrtg/performance_ntp.php

PC Narvik has just been rebooted (09:35 UTC), and is within 0.25ms.  PC 
Puffin was switch on at about 04:30 UTC after being off overnight and was 
within about 0.5ms.  These are LAN-synced systems, running Windows, and 
tuned for that environment.  You would likely not achieve the same either 
ultimate accuracy or settling time when working from a set of Internet 
servers, or you have an unfortunate combination of OS, motherboard, 
application set, or CPU power-saving activation.

Cheers,
David 




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