[ntp:questions] Why does NTPD precision decline when using the HPET ves. the Performance Counter?

Charles Elliott elliott.ch at comcast.net
Thu Sep 4 10:58:12 UTC 2014


When the platform clock is changed from the 
performance counter (freq: 3.554 MHz) to the
HPET (freq: 14.318 MHz) the NTP protocol
precision declines from -22 to -20.  This
occurs both in versions 4.2.7p442 at 1.2483-o May 09 10:14:35.18
and 4.2.7p467 at 1.2483-o Aug 28 12:01:29.42.  
The NTPD_USE_INTERP_DANGEROUS=1 environment 
variable is set.  Here are the relevant messages from 
the Event Log:

8/23/2014 3:34:59 PM ntpd 4.2.7p442 at 1.2483-o May 09 10:14:35.18 (UTC-04:00)
2014  (1): Starting   
8/23/2014 3:34:59 PM Command line: G:\Program Files (x86)\NTP\bin\ntpd.exe
-U 3 -M -g -c G:\NTP_Conf\ntp.conf   
8/23/2014 3:34:59 PM Raised to realtime priority class   
8/23/2014 3:34:59 PM Clock interrupt period 15.625 msec (startup slew -0.7
usec/period)   
8/23/2014 3:34:59 PM Performance counter frequency 3.554 MHz
8/23/2014 3:34:59 PM proto: precision = 0.200 usec (-22)   
8/23/2014 3:34:59 PM proto: fuzz beneath 0.100 usec   


8/23/2014 3:52:37 PM ntpd 4.2.7p442 at 1.2483-o May 09 10:14:35.18 (UTC-04:00)
2014  (1): Starting   
8/23/2014 3:52:37 PM Command line: G:\Program Files (x86)\NTP\bin\ntpd.exe
-U 3 -M -g -c G:\NTP_Conf\ntp.conf   
8/23/2014 3:52:37 PM Raised to realtime priority class   
8/23/2014 3:52:37 PM Clock interrupt period 15.625 msec   
8/23/2014 3:52:37 PM Performance counter frequency 14.318 MHz   
8/23/2014 3:52:38 PM proto: precision = 0.800 usec (-20)   
→ → No "proto: fuzz beneath ... " message ← ←

9/4/2014 5:57:34 AM ntpd 4.2.7p467 at 1.2483-o Aug 28 12:01:29.42 (UTC+01:00)
2014  (1): Starting   
Repeats exact same sequence as on 8/23/2014 3:52:37 PM

Is this the way it should be, that the protocol precision
declines when the clock is more than 4 times faster?

Charles Elliott



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