[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
kaukasoina708n8s6l3g0 at sci.fi
Tue Jan 22 11:37:06 UTC 2008
Unruh <unruh-spam at physics.ubc.ca> wrote:
>After I collect more data on steady state, I will rerun startups both with
>no drift file and a bad drift file to see how fast the convergence is with
On recent Linux kernels, I think the drift file is always bad after reboot.
HZ=100, no dynamic ticks aka tickless system (CONFIG_NO_HZ not set). I think
I even tried with a kernel command line option lpj= but it didn't help.
If the system is rebooted, ntpd stabilizes to a new different drift value.
With a bad or missing drift file, time set with ntpdate, ntpd can soon take
the offset to a 100 or 200 ms error for a long time.
If you are using Linux and are experimenting with these, please try
something like this which has given me good results (a coarse calibration of
the drift file during boot before starting ntpd):
# remember to stop ntpd first if running
# reset frequency offset to zero
adjtimex -f 0
# calibrate clock rate during $TIME seconds
ntpdate -sb $NTPSERVER
ADJUST=$(ntpdate -b $NTPSERVER | sed 's/.*offset \(.*\) sec.*/\1/')
# ntpdate adjusted $ADJUST seconds
FREQUENCY=$(echo "scale=3; $ADJUST * 1000000 / $TIME" | bc)
# reset the drift file and start ntpd
echo $FREQUENCY > $DRIFTFILE
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