[time-nuts] FreeBSD, NetBSD, or Minix-III?
M. Warner Losh
imp at bsdimp.com
Mon May 18 15:24:14 UTC 2009
In message: <C636B079.7D49%James.P.Lux at jpl.nasa.gov>
"Lux, James P" <james.p.lux at jpl.nasa.gov> writes:
: I also wouldn't have the low order counter count nanoseconds, or even set it
: up as seconds/subseconds.
I'd echo this, since you are artificially limiting the clocks that are
input to having a period of an exact number of nanoseconds. This
rounding could lead to systematic errors that would lead to a higher
noise in the measurements.
A simple counter is more flexible. It allows for a number of
additional algorithms to be applied to the raw time measurements to
account for drift in the underlying oscillator. Phk's timecounters
allow for this. They assume a time source that is free-running. It
can be measured against a known better source to improve its accuracy
(which is what ntpd does). This allows one to correct over time for,
say, the relatively crappy internal oscillator found in most PCs. The
nice thing about timecounters is they allow hardware as described in
this thread to replace the underlying system hardware.
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