[ntp:questions] Quick sync between two computers not connected to the internet

unruh unruh at wormhole.physics.ubc.ca
Sat Mar 20 00:22:11 UTC 2010

On 2010-03-19, Dave Hart <davehart at gmail.com> wrote:
> On Mar 19, 18:21?UTC, a xs4all user wrote:
>> Of course you should not configure them to follow another in both directions.
>> When the second computer follows the first, there is nothing instable
>> or wrong going on. ?You can even pick the computer that has the most
>> stable clock before configuring them.
> That's tricky without internet or other access to a stable frequency
> source.  The machines in question do not and never will have access to
> the internet.

So just pick one randomly, or because you like its colour

>> He only requires the time to be the same on both computers. ?There is
>> nothing that a GPS receiver will add to the system to make this easier
>> than without it. ?But of course it adds complexity and requirements.
> There's one thing -- irrelevant if w32 time sync is close enough, but
> ntpd has a 500ppm limit on frequency correction.  Depending on the
> number of machines (two here) and each machine's normal frequency
> error, it may be necessary to get the source machine's frequency
> closer to correct to bring it in the range the other can correct.
> This is unlikely to be a problem with a couple of machines each within
> 125ppm, for example, but get unlucky with a few hundred ppm positive
> on one and negative on the other, or add more machines to the brew,
> and your chances of having problems using an errant frequency standard
> (the source machine's uncorrected clock) go up.

That 500PPM was chosen as a sufficiently large value that no standard
machine should come close. For a while the linux kernel time
standardizing routine really screwed up and you would have machines
which were 100-200 PPM out. But that has now I believe been fixed. Ie,
this should not be a problem.

More information about the questions mailing list