[ntp:questions] ntpd and database servers

unruh unruh at wormhole.physics.ubc.ca
Fri Jan 15 00:06:49 UTC 2010

On 2010-01-14, nodata <lsof at nodata.co.uk> wrote:
> Am 2010-01-14 20:38, schrieb Rob:
>> You could stop ntpd, and then use the adjtimex program to set the
>> clock rate to a slower value, so that the systems will lose time.
>> (using the --tick option)
>> Calculate how long it is going to take to arrive at the correct time
>> using the setting you decide on, and then come back to the system
>> (probably put an alert in your electronic calender) to check that the
>> time is slightly behind correct time.
>> Then reset the adjtimex setting to the normal rate and restart ntpd.
>> It will probably jump forward a little at that moment but that should
>> not matter.
> Thanks for this.
> Would you mind explaining the link between CONFIG_HZ and the tick value? 
> The example in the man page is not clear (it uses the same tick value as 
> I have on a CONFIG_HZ=1000 box). Is the statement "Increasing val by 1 
> speeds up the system clock by about 100 ppm, or 8.64 sec/day." always 
> true, whatever the CONFIG_HZ value?

man adjtimes

  -t val, --tick val
              Set the number of microseconds that should be added to the
               system time for each kernel tick interrupt.  For a kernel with
              USER_HZ=100, there are supposed to be 100 ticks per second, so
              val should be close to 10000.  Increasing val by 1 speeds up the
              system clock by about 100 ppm, or 8.64 sec/day.  tick must be in
              the range 900000/USER_HZ...1100000/USER_HZ.  If val is rejected
              by the kernel, adjtimex will determine the acceptable range
              through trial and error and print it.  (After completing the
              search, it will restore the original value.)

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