>
>
> The problem for ntp is that ntp takes a long time to recover from a bad
> drift value.
>
This seems to have been an issue since I started using ntp, more than 10 years
ago. I am surprised that it is not fixed.
A simple test on linux with a modern version of ntp: Here is the normal state
of this R-PI
Thu Dec 5 09:34:13 CET 2013
mike@raspberrypi ~ $ sudo cat /var/lib/ntp/ntp.drift
-36.772
mike@raspberrypi ~ $ ls -l /var/lib/ntp/ntp.drift
-rw-r--r-- 1 root root 8 Dec 5 08:51 /var/lib/ntp/ntp.drift
mike@raspberrypi ~ $ ntpq -pn |grep \*
mintc=3, offset=-0.169517, frequency=-37.191, sys_jitter=0.333279,
offset with this server is fairly stable at 1-300 microseconds, sometimes
better.
So now stop ntpd , stick a silly value in the drift file and restart.
root@raspberrypi:/home/mike# echo "-256.666" > /var/lib/ntp/ntp.drift
root@raspberrypi:/home/mike# cat /var/lib/ntp/ntp.drift
-256.666
root@raspberrypi:/home/mike# /etc/init.d/ntp start
Starting NTP server: ntpd.
root@raspberrypi:/home/mike# ntpq -c rv
associd=0 status=0615 leap_none, sync_ntp, 1 event, clock_sync,
version="ntpd 4.2.7p319@1.2483 Tue May 28 11:26:22 UTC 2013 (2)",
processor="armv6l", system="Linux/3.2.27-pps", leap=00, stratum=2,
precision=-19, rootdelay=14.258, rootdisp=202.121, refid=145.238.203.14,
reftime=d64aba43.dfdbb690 Thu, Dec 5 2013 9:39:31.874,
clock=d64aba54.494fd9c7 Thu, Dec 5 2013 9:39:48.286, peer=2675, tc=6,
mintc=3, offset=3.357234, frequency=-256.666, sys_jitter=1.622350,
clk_jitter=2.342, clk_wander=0.000
So we have picked up the drift and are using it as is, no verification.
root@raspberrypi:/home/mike# ntpq -pn
...
*145.238.203.14 .TS-3. 1 u 44 64 1 14.258 3.357 1.622
...
So iburst got us a reasonable start point. Now lets see how it evolves:
oot@raspberrypi:/home/mike# while true; do date; ntpq -pn |grep \*;ntpq -c rv
|grep frequency; ls -l /var/lib/ntp/ntp.drift;cat /var/lib/ntp/ntp.drift; sleep
60; done
Thu Dec 5 09:46:00 CET 2013
*145.238.203.14 .TS-3. 1 u 62 64 77 14.258 3.357 11.413
mintc=3, offset=16.974005, frequency=-256.666, sys_jitter=11.412631,
-rw-r--r-- 1 root root 9 Dec 5 09:38 /var/lib/ntp/ntp.drift
-256.666
three samples later ,
Thu Dec 5 09:49:01 CET 2013
*145.238.203.14 .TS-3. 1 u 39 64 377 14.270 13.392 25.459
the offset multiplies by three
mintc=3, offset=16.974005, frequency=-256.666, sys_jitter=25.458613,
-rw-r--r-- 1 root root 9 Dec 5 09:38 /var/lib/ntp/ntp.drift
-256.666
Thu Dec 5 09:50:02 CET 2013
*145.238.203.14 .TS-3. 1 u 32 64 377 14.272 64.913 38.415
then more than 20 times
mintc=3, offset=64.912586, frequency=-224.970, sys_jitter=38.415064,
-rw-r--r-- 1 root root 9 Dec 5 09:38 /var/lib/ntp/ntp.drift
-256.666
Thu Dec 5 09:51:02 CET 2013
*145.238.203.14 .TS-3. 1 u 25 64 377 14.272 64.913 34.083
mintc=3, offset=64.912586, frequency=-224.970, sys_jitter=34.083058,
-rw-r--r-- 1 root root 9 Dec 5 09:38 /var/lib/ntp/ntp.drift
-256.666
Thu Dec 5 09:52:02 CET 2013
*145.238.203.14 .TS-3. 1 u 19 64 377 14.242 78.513 37.945
and it gets worse - note that we still think this is a good source
mintc=3, offset=78.512782, frequency=-214.937, sys_jitter=37.944744,
-rw-r--r-- 1 root root 9 Dec 5 09:38 /var/lib/ntp/ntp.drift
-256.666
Thu Dec 5 09:53:03 CET 2013
*145.238.203.14 .TS-3. 1 u 10 64 377 14.242 78.513 30.074
mintc=3, offset=78.512782, frequency=-214.937, sys_jitter=30.073729,
-rw-r--r-- 1 root root 9 Dec 5 09:38 /var/lib/ntp/ntp.drift
-256.666
Our worst state is at 10:03, 30 minutes after the start up. The real time
frequency value is decreasing but not reflected to the file. This is an issue
as an admin blindly restarting ntp after noticing crappy offsets will hit the
same wall again.
The file gets updated after 1Hr, at
Thu Dec 5 10:39:21 CET 2013
*145.238.203.14 .TS-3. 1 u 25 64 377 12.834 37.836 8.350
mintc=3, offset=37.835862, frequency=-88.963, sys_jitter=8.349705,
-rw-r--r-- 1 root root 8 Dec 5 10:39 /var/lib/ntp/ntp.drift
-88.963
The rate of convergence is getting quicker but we don't get back to a good
state until nearly 3Hrs:
Thu Dec 5 12:20:02 CET 2013
*145.238.203.14 .TS-3. 1 u 28 64 377 12.979 0.287 0.693
mintc=3, offset=0.287015, frequency=-38.180, sys_jitter=0.693195,
-rw-r--r-- 1 root root 8 Dec 5 11:39 /var/lib/ntp/ntp.drift
-41.923
And the "normal" drift is reached around 4hrs after the restart.
Thu Dec 5 13:30:30 CET 2013
*145.238.203.14 .TS-3. 1 u 60 64 377 12.882 0.134 0.058
mintc=3, offset=0.134499, frequency=-37.307, sys_jitter=0.057602,
-rw-r--r-- 1 root root 8 Dec 5 12:39 /var/lib/ntp/ntp.drift
-37.928
I am sure that a much faster convergence could be achieved with a little
thought, even if it meant a little ringing.
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