[ntp:questions] once 'ntpd' stops, does the drift file value continue to get used for clock adjustments?

Dave Hart davehart at gmail.com
Fri Mar 18 13:45:14 UTC 2011

On Fri, Mar 18, 2011 at 8:38 AM, Martin Burnicki
<martin.burnicki at meinberg.de> wrote:
> David Woolley wrote:
>> Windows will probably retain the last correction, but this may include a
>>   high frequency component.
> AFAIK the Windows port of ntpd restores the standard tick value when it
> terminates, so the drift compensation isn't in effect anymore thereafter.

I believe that was the case for 4.2.4.  With 4.2.6 and later, the last
correction is left in place unless you happen to stop ntpd just after
a leap second.  You can see evidence (in unfamiliar units) in the log
at ntpd restart:

7 Mar 07:56:48 ntpd[4196]: Clock interrupt period 15.600 msec (startup
slew 18.9 usec/period)

Here's the code.  Note the function name is misleading, it is called
at ntpd shutdown similarly to how init_winnt_time() is called during


	 * If we're in the 2-second slew right after a leap second,
	 * we don't want to continue that extreme slew, in that case
	 * disable our slewing and return clock discipline to the
	 * kernel.  Similarly if we are not yet synchronized,
	 * our current slew may not be a good ongoing trim.
	 * Otherwise, our leave in place the last SetSystemTimeAdjustment
	 * as an ongoing frequency correction, better than nothing.
	 * TODO:
	 * Verify this will not call SetSystemTimeAdjustment if
	 * ntpd is running in ntpdate mode.
	if (sys_leap == LEAP_NOTINSYNC || ls_time_adjustment)
		SetSystemTimeAdjustment(0, TRUE);	

	 * Read the current system time, and write it back to
	 * force CMOS update, only if we are exiting because
	 * the computer is shutting down and we are already
	 * synchronized.
	 if (ntservice_systemisshuttingdown() && sys_leap != LEAP_NOTINSYNC) {
			msyslog(LOG_NOTICE, "system is shutting down, CMOS time reset.");

Dave Hart

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