[ntp:questions] drifting on crystal

David Monniaux monniauxtrash at quatramaran.ens.fr
Wed Jan 12 22:42:59 UTC 2005

We have a scientific experiment in a remote location abroad where good 
timekeeping is important. This experiment is monitored by a computer 1) 
connected to the Internet using a DSL line (which goes down for periods 
at times) 2) has a GPS receiver (which has problems of its own).

When we are unlucky, the GPS has its "issues" within the same period 
that DSL has its problems, and thus the computer becomes unsynchronized 
with respect to precision clocks.

I now wonder about the timekeeping in those bad periods. Questions:

1) While ntp works well, it can presumably have an idea of the amount by 
which the system clock tends to drift (say, "the system clock is 10^-6 
slower than it should be") compared to reference clocks. Does ntp use 
this information to compensate the clock when the reference clocks are 

2) The machine is in constant use (no powercycles) and within a known 
temperature range. Can we get an order of magnitude of the drift of the 
system clock?

For information, it's running Linux kernel 2.4.22.


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