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

Thierry MARTIN thierry-martin at ifrance.com
Tue Feb 19 16:45:59 UTC 2008

Hi Serge,

My investigations might lead me to choose your solution ;-).

The system I have is a (trans)portable machine. I wonder wether these 
parameters will be ok, wherever I place it (what is the influnce of 
temperature differences or things like that)?

Thierry MARTIN

Serge Bets a écrit :
> Hello again Thierry,
>  On Tuesday, February 12, 2008 at 10:58:32 +0100, Thierry MARTIN wrote:
>> 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)?
> Absolutely normal: that's only 23 PPM, nothing. I have seen machines
> that do better, and some that do much worse. As long as it's stable,
> 2 secs/day every day, and below 500 PPM, it's zeroifiable via the kernel
> clock frequency setting. Even above 500 PPM, with help from --tick.
>> I tryied "adjtimex -a" which gives unreliable results
> This --adjust thing is quite reliable, easely to the single PPM (less
> than 0.1 secs/day), provided you already carefully calibrated its
> reference: The RTC, via hwclock and /etc/adjtime.
> There are plenty of other methods to get an accurate kernel clock
> frequency. I'll propose one: run ntpd on the machine during some hours
> or a day, with a good connection to (a) good server(s). Then the
> following days restore the wanted kernel frequency via:
> | # ntptime -f $(cat /var/lib/ntp/ntp.drift) > /dev/null
> Or via:
> | # adjtimex --frequency $(awk '{printf "%.0f", $1 * 65536; exit}' /var/lib/ntp/ntp.drift)
> Cordialement, Serge.

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