Need better is_better_time_interpolator() algorithm

[Alex Williamson <alex.williamson@hp.com>]

Hi,

   In playing with an HPET device, I noticed that
kernel/timer.c:is_better_time_interpolator() is completely non-symmetric
in the timer it returns.  The test is simply:

return new->frequency > 2*time_interpolator->frequency ||
 (unsigned long)new->drift < (unsigned long)time_interpolator->drift;

Given two timers:

(a) 1.5GHz, 750ppm
(b) 250Mhz, 500ppm

the resulting "better" timer is completely dependent on the order
they're passed in.  For example, (a),(b) = (b); (b),(a) = (a).

   What are we really looking for in a "better" timer?  There are at
least 4 factors that I can think of that seem important to determining a
better clock:

      * resolution (frequency)
      * accuracy (drift)
      * access latency (may be non-uniform across the system?)
      * jitter (monotonically increasing)

How can we munge these all together to come up with a single goodness
factor for comparison?  There's probably a thesis covering algorithms to
handle this.  Anyone know of one or have some good ideas?  Thanks,

	Alex

-- 
Alex Williamson                             HP Linux & Open Source Lab

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