"unruh" <[email protected]> wrote in message
news:[email protected]...
On 2011-04-18, David J Taylor <[email protected]>
wrote:
"C BlacK" <[email protected]> wrote in message
news:[email protected]...
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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