[ntp:questions] no ntp synchronisation: 2s to 6s time shift !

Martin Burnicki martin.burnicki at meinberg.de
Tue Feb 12 10:27:40 UTC 2008


Thierry,

Thierry MARTIN wrote:
> Hello,
> 
> Can anyone tell me if a time shift greater than 2s per day is ""normal""
> on a linux system that has no "external" synchronisation (ntp)?
> 
> I tryied "adjtimex -a" which gives unreliable results: it worked fine on
>    one machine (less than 1s shift / day) and badly on another one
> (several seconds shift / day)
> 
> Any info about this subject would help me.

Which kernel version are you running? Starting with ~2.6.22 a new clock
model has made its way into the Linux kernel. There are different modules
which can use different hardware timers for timekeeping, and there seem to
be some problems with certain modules on certain hardware (i.e. chipsets on
the motherboard).

To list available clock sources under kernel ~2.6.22 or newer:

# cat /sys/devices/system/clocksource/clocksource0/available_clocksource
hpet acpi_pm pit jiffies tsc

Check which clocksource is currently being used:

# cat /sys/devices/system/clocksource/clocksource0/current_clocksource
tsc

Change the clock source:

# echo tsc > \
/sys/devices/system/clocksource/clocksource0/current_clocksource

For a test it is also possible to override the default clock source using a
boot parameter, e.g.:

clocksource=jiffies

You may want to try if some other of the available modules does a better job
in timekeeping. If the active module doesn't work correctly then the
frequency drift often exceeds the maximum drift ntpd can handle.


Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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