[ntp:questions] What prevents continuous time within an operating system ?

MAYER Hans mayer at iiasa.ac.at
Sat Jan 16 15:26:21 UTC 2016

Dear All,

I connected a GPS based 1PPS signal to a Banana Pi as stratum 0 source. 
I watched the offset in the peerstats file and it goes up and down sometimes. 
So I assumed this happens due to the fading of the signal from the GPS satellites. 
Later on I installed a second 1PPS signal based on a rubidium frequency standard. 
So I realized that both time-stamps have the same issue. See here 
https://farm2.staticflickr.com/1642/24214179542_89e2b28d21_b.jpg is rubidium, is GPS. is the time source, is configured "noselect". 
As you can see offset is typically between +/- 5 micro seconds 
but currently at around 18:00 h the offset is 40 micro seconds.
the same behavior with the GPS signal. 
Both fit very well. So it's not the time source, it must be the operating system. 
And of course this doesn't happen always at the same time, it's different. 

I am not sure if this is related to the Banana Pi and it's operating system.
OS is Bananian OS which is actually a Debian OS with kernel  3.4.108+ 
It has 1GB of RAM where almost all is free. Nothing else as NTPD and GPSD 
is running. 

I also wonder why NTP needs almost 2 hours to find back to the "normal" offset 
if it does read the time stamp each 32 seconds ? 

Kind regards


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