[ntp:questions] NTP not syncing

unruh unruh at invalid.ca
Wed Dec 4 23:38:17 UTC 2013

On 2013-12-04, antonio.marcheselli at gmail.com <antonio.marcheselli at gmail.com> wrote:
>> It is very common, IF you are running on Linux!
>> The base frequency is recalculated each time you restart, which means 
>> that steps of 100-200 ppm from one reboot to the next can be expected.
> Hi,
> Yes, it's Linux.
> But I did not reboot the server, only the service.
> Anyway, I thought the drift was showing how much the BIOS clock was faster or slower than the reference clock.

No. The bios clock is ONLY used on startup to set the intial time of the
system clock on bootup. It is never used thereafter. The system clock is
a counter (eg counting the number of processor instruction cycles,
counting the number of HPET cycles etc) but those counts have to be
calbrated to time. You have to know how many of those counts correspond
to a second. The computer does that at bootup (probablyusing the bios
clock, but I was never clear exactly how that calibration was done), and
on linux that used to be flakey (older Linux were fine, and then
something got broken) I think it is now fixed, but am not sure. 

> What do you mean when you say that the base frequency is recalculated?
> I must be very slow here, I've been asking about the drift but it seems I still haven't grasped the meaning of it...

The drift is the difference between how many seconds the computer says
has elapsed vs the number of seconds that UTC says have elapsed
expressed as PPM ( (#computer seconds/#UTC seconds -1)*1000000)

> Thanks
> Antonio

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