[LEAPSECS] private smear goes public

[Hal Murray]

> Could someone provide an introduction to how the rates of these reference
> clocks are adjusted? What's going on inside the black box? 
...
> Can the frequency of the crystal (or whatever) oscillators be adjusted by
> applying some bias voltage? 

It's all done by software.

Each CPU has a nominal clock frequency and some counter that ticks at that 
rate.

The kernel timekeeping routine does something like:
  newcycles = readMagicRegister()
  cycles = newcycles - oldcycles
  oldcycles = newsycles
  newtime = oldtime + cycles*timePerCycle
  oldtime = newtime

The trick is that you need a lot of low bits in timePerCycle.  It's not 
simple integer arithmetic.

The kernel has a system call that ntpd uses to tweak the value of 
timePerCycle.  It's the ntpd drift parameter.  Normally it's used to correct 
for the crystal being slightly off from the nominal value due to 
manufacturing tolerances.  It also tracks minor changes due to temperature 
and aging.

Fudging it slightly to implement a leap smear fits well within reasonable 
numbers as long as the smear is spread over a long enough time slot.  20 
hours is long enough.  ntpd can't tell the difference between a leap smear 
and a temperature shift.



-- 
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