At 3:59 PM +0000 2/19/11, Ian Batten wrote:
On 19 Feb 2011, at 15:41, Gerard Ashton wrote:
On 2/19/2011 10:24 AM, Joe Gwinn wrote, in part:
I have not been following the proposal in detail, but a key issue
to the POSIX community is that their timescale must be
implementable in a totally isolated machine, one having no GPS or
internet access.
There are other requirements as well. This was discussed at
length on the Time Nuts reflector, until Tom kicked the thread
over to Leap Secs.
Joe Gwinn
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I think a description of the totally isolated machine requirement
would have to be specified
to make progress. How long would the machine be expected to keep
good time, what is the acceptable
tolerance for time of day, and what technology would be used to keep time?
With respect to leapseconds, a debate starting from a completely
isolated computer is almost meaningless. Leapseconds represent a
one part in ~5x10^7 divergence (0.02ppm) from TAI+initial offset.
Even if you can find a free-running crystal oscillator of that
precision given constant temperature, the environment in a piece of
computer hardware varies in terms of temperature sufficient to throw
that off by more than 0.02ppm, and (whisper it!) computers have been
known to drop clock interrupts at a rate at least comparable to 1 in
5x10^7. Show me a computer that can run its operating system clock,
ie the return value from time(), such that it gains or loses
significantly less than a second over the course of a year and we
can talk about the semantics of Posix time relative to leap seconds
in an isolated environment.
Now it's possible that one scenario is a machine which is hooked to
a Caesium-Rubidium clock, but has no means of obtaining DUT1 or leap
second notifications, but nonetheless needs to track UT1 or UTC to
high precision. I think that use-case needs to be articulated
before it's used to hold this debate hostage, because I'm struggling
to think what it is.
This is all true, but solves a problem that the POSIX Committee
neither understood nor cared about.
This was beaten to death on the Time Nuts reflector in the thread
"Leap seconds and POSIX" around January 2009.
Joe Gwinn
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