[ntp:questions] how long does it take ntpd to sync up

unruh unruh at wormhole.physics.ubc.ca
Sat Aug 27 19:33:50 UTC 2011

On 2011-08-27, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Sat, Aug 27, 2011 at 9:49 AM, C BlacK <rblak at non.net> wrote:
>> I read somewhere that it takes an ntpd 24 hours to truly sync up to a
>> stratum 2 master.
> Use some logic.   Assume you want to set a mechanical clock's speed by
> moving the "slow/fast" lever and you can see the time only to the nearest

Ah yes, but why a) would you want to only adjust it that way, an b) why
not do a fit and figure outexactly what the rate of the clock is and how
far out it is, and correct that, rather then simply "shove the rate a
bit higher if you see the clock time is behind time now, and a bit slower if
you see the clock is ahead." (which is what ntp does). 

> second by eye.  If you want the clock to loose/gain one second per day then
> it will take you one day (at best) to adjust the time as it would take that
> long to detect an error.   It you want to adjust the rate such that it only
> gains one second per month it would take a month before you'd know if you
> got it right.  You have to remember that NTP works be adjusting the rate,
> faster or slower and tried hard to never "jump" the time.   The only why for
> anyone to know if the rate is right is to wait, greater accuracy requires
> longer a wait.

But ntpd has no compunction about jumping the time if it is out by 125
ms. It does not try very hard not to jump the time at all. 

> NTP works kind of like this.  Small errors take a long time to detect and
> smaller errors take even longer to detect.  So up to a point NTP's error

no, an error in offset of 1 usec can be detected immediately if you have
a good reference (and almost never if you do not) . And an error in rate of 1usec/s can be detected easily
within a minute. 

> gets smaller the longer you wait.  The details depend on your Internet
> connection and the distance to the server(s).  For most people a "few" hours
> gets you "close" and you are almost as good as you will be after a half day.
>    After 24 hours improvement is unlikely.
> So the conservative answer is "24 hours" but for most practical purposes you
> are "good enough for file system time stamps and the like" after 1 or 2
> hours.

Depends on your accuracy requirements. 


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