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Following a reboot on my Windows-7 RC test PC at 12:30 UTC yesterday, I noticed that the NTP offset as seen on my MRTG graph was a lot more unstable than I expected.  However, I thought it might stabilise, so I left it.

However, even some five hours later, the offset continued to show a high level of variation, so I stopped and restarted NTP without changing any parameters.  Rather to my surprise, normal stable operation resumed, with a gradual drift towards a zero mean offset, and with a much lower level of offset variation.

Here are the NTP entries extracted from the Event Log, the times are UTC+1.  They show that when NTP was first started after boot, it saw the System time precision as 15.625msec, but after the stop and restart of the NTP service it saw the System time precision as 0.977msec.  With these different values, a different route in the code was taken (perhaps the interpolation was enabled when the 15.625msec value was recognised), leading to the poorer performance.

Level,Date and Time,Source,Event ID,Task Category
Information,13/06/2009 19:45:31,NTP,3,None,Detected positive leap second announcement for 2009-07-01 00:00:00 UTC 
Information,13/06/2009 18:37:11,NTP,3,None,"synchronized to 192.168.0.2, stratum 1 "
Information,13/06/2009 18:37:05,NTP,3,None,frequency initialized 6.693 PPM from C:\Tools\NTP\etc\ntp.drift 
Information,13/06/2009 18:37:05,NTP,3,None,"Listening on interface #4 IP Interface 4, 169.254.0.109#123 Enabled "
Information,13/06/2009 18:37:05,NTP,3,None,"Listening on interface #3 IP Interface 3, 192.168.0.8#123 Enabled "
Information,13/06/2009 18:37:05,NTP,3,None,"Listening on interface #2 IP Interface 2, 192.168.238.238#123 Enabled "
Information,13/06/2009 18:37:05,NTP,3,None,"Listening on interface #1 Loopback Interface 1, 127.0.0.1#123 Enabled "
Information,13/06/2009 18:37:05,NTP,3,None,"Listening on interface #0 wildcard, 0.0.0.0#123 Disabled "
Information,13/06/2009 18:37:05,NTP,3,None,precision = 976.600 usec 
Information,13/06/2009 18:37:05,NTP,3,None,using native clock directly 
Information,13/06/2009 18:37:05,NTP,3,None,"System time precision 0.977 msec, min. slew 6.400 ppm/s "
Information,13/06/2009 18:37:05,NTP,3,None,Clock interrupt period 15.625 msec (startup slew 0.1 usec/period) 
Information,13/06/2009 18:37:05,NTP,3,None,Performance counter frequency 3.580 MHz 
Information,13/06/2009 18:37:05,NTP,3,None,"MM timer resolution: 1..1000000 msec, set to 1 msec "
Information,13/06/2009 18:37:05,NTP,3,None,Raised to realtime priority class 
Information,13/06/2009 18:37:05,NTP,3,None,ntpd 4.2.4p6@DLH-QPC-o Mar 10 15:23:14.36 (UTC) 2009 (230) 
Information,13/06/2009 18:37:03,NTP,3,None,ntservice: The Network Time Protocol Service has stopped. 
Information,13/06/2009 15:00:04,NTP,3,None,"synchronized to 192.168.0.2, stratum 1 "
Information,13/06/2009 14:58:59,NTP,3,None,"synchronized to 192.168.0.7, stratum 1 "
Information,13/06/2009 14:28:08,NTP,3,None,Detected positive leap second announcement for 2009-07-01 00:00:00 UTC 
Information,13/06/2009 13:44:36,NTP,3,None,"synchronized to 192.168.0.2, stratum 1 "
Information,13/06/2009 13:36:55,NTP,3,None,interp_time thd 6ec mean -0.0614 spread 0.0945 msec 
Information,13/06/2009 13:36:28,NTP,3,None,interp_time thd 198 mean -0.0528 spread 0.9501 msec 
Information,13/06/2009 13:35:58,NTP,3,None,time reset +5.930660 s 
Information,13/06/2009 13:35:56,NTP,3,None,HZ 64.000 using 43 msec timer 23.256 Hz 64 deep 
Information,13/06/2009 13:35:50,NTP,3,None,"synchronized to 192.168.0.2, stratum 1 "
Information,13/06/2009 13:35:44,NTP,3,None,frequency initialized 4.859 PPM from C:\Tools\NTP\etc\ntp.drift 
Information,13/06/2009 13:35:44,NTP,3,None,"Listening on interface #4 IP Interface 4, 169.254.0.109#123 Enabled "
Information,13/06/2009 13:35:44,NTP,3,None,"Listening on interface #3 IP Interface 3, 192.168.0.8#123 Enabled "
Information,13/06/2009 13:35:44,NTP,3,None,"Listening on interface #2 IP Interface 2, 192.168.238.238#123 Enabled "
Information,13/06/2009 13:35:44,NTP,3,None,"Listening on interface #1 Loopback Interface 1, 127.0.0.1#123 Enabled "
Information,13/06/2009 13:35:44,NTP,3,None,"Listening on interface #0 wildcard, 0.0.0.0#123 Disabled "
Information,13/06/2009 13:35:44,NTP,3,None,precision = 1.900 usec 
Information,13/06/2009 13:35:41,NTP,3,None,HZ 64.000 using 43 msec timer 23.256 Hz 64 deep 
Information,13/06/2009 13:35:41,NTP,3,None,"System time precision 15.625 msec, min. slew 6.400 ppm/s "
Information,13/06/2009 13:35:41,NTP,3,None,Clock interrupt period 15.625 msec 
Information,13/06/2009 13:35:41,NTP,3,None,Performance counter frequency 3.580 MHz 
Information,13/06/2009 13:35:41,NTP,3,None,"MM timer resolution: 1..1000000 msec, set to 1 msec "
Information,13/06/2009 13:35:41,NTP,3,None,Raised to realtime priority class 
Information,13/06/2009 13:35:41,NTP,3,None,ntpd 4.2.4p6@DLH-QPC-o Mar 10 15:23:14.36 (UTC) 2009 (230) 
Information,13/06/2009 13:34:39,NTP,3,None,ntservice: T

Using an updated version.....
  
Level,Date and Time,Source,Event ID,Task Category
Information,14/06/2009 10:40:24,NTP,3,None,"synchronized to 192.168.0.2, stratum 1 "
Information,14/06/2009 10:39:59,NTP,3,None,time reset +0.977355 s 
Information,14/06/2009 10:39:58,NTP,3,None,"synchronized to 192.168.0.2, stratum 1 "
Information,14/06/2009 10:39:42,NTP,3,None,frequency initialized 4.919 PPM from C:\Tools\NTP\etc\ntp.drift 
Information,14/06/2009 10:39:42,NTP,3,None,"Listening on interface #4 IP Interface 4, 169.254.0.109#123 Enabled "
Information,14/06/2009 10:39:42,NTP,3,None,"Listening on interface #3 IP Interface 3, 192.168.0.8#123 Enabled "
Information,14/06/2009 10:39:42,NTP,3,None,"Listening on interface #2 IP Interface 2, 192.168.238.238#123 Enabled "
Information,14/06/2009 10:39:42,NTP,3,None,"Listening on interface #1 Loopback Interface 1, 127.0.0.1#123 Enabled "
Information,14/06/2009 10:39:42,NTP,3,None,"Listening on interface #0 wildcard, 0.0.0.0#123 Disabled "
Information,14/06/2009 10:39:42,NTP,3,None,precision = 976.500 usec 
Information,14/06/2009 10:39:42,NTP,3,None,using native clock directly 
Information,14/06/2009 10:39:42,NTP,3,None,"System time precision 0.977 msec, min. slew 6.400 ppm/s "
Information,14/06/2009 10:39:42,NTP,3,None,Clock interrupt period 15.625 msec 
Information,14/06/2009 10:39:42,NTP,3,None,Performance counter frequency 3.580 MHz 
Information,14/06/2009 10:39:42,NTP,3,None,"MM timer resolution: 1..1000000 msec, set to 1 msec "
Information,14/06/2009 10:39:42,NTP,3,None,Raised to realtime priority class 
Information,14/06/2009 10:39:42,NTP,3,None,ntpd 4.2.4p6@DLH-QPC-o May 30 3:58:32.88 (UTC) 2009 (273) 
Information,14/06/2009 10:37:31,NTP,3,None,ntservice: The Network Time Protocol Service has stopped. 

Here are the the offset, frequency and jitter plots for the last two and a bit days.

 
Copyright © David Taylor, Edinburgh   Last modified: 2015 Jan 18 at 09:32