[time-nuts] Using a frequency synthesizer replacement for motherboard oscillator

David I. Emery die at dieconsulting.com
Sat Dec 1 04:39:44 UTC 2012

On Fri, Nov 30, 2012 at 09:58:29PM -0500, Bob Camp wrote:
> Hi

> The problem just the clock it's also the operating system. If it's not
> designed with timing in mind (= it's an RTOS at some level) then you
> will have sloppy timing. Counters can help, but they are not the entire
> solution. If your email (or anti-virus or ...) program can pop up and
> monopolize the cpu for a chunk of a second (as in 10's or 100's of ms), 
> precision timing isn't going to work very well. There's only so much you
> can do after the fact. If the pps edge was supposed to go out 27 ms ago,
> and you only got control back now, you are out of luck. 

	No doubt that you get into a sort of philosophic meaning of 
what-really-is-now relativity here... if parts of the kernel know what
time it is quite precisely but other parts and most user programs are
only loosely aware of and only able to react to it post facto by large 
and jittery intervals, what is the meaning of microsecond or even ns
level OS time sync ?

	Most modern kernels *internally* have at least some degree of
fairly serious real time high priority tight deadline stuff going on -
and API hooks for accessing it available - the degree to which this is
exposed and visible to or usable by user space threads varies a lot, and
correctly using this stuff always requires pretty deep skill and
thought.  Not for the faint of heart or inexperienced.   Very easy to
make subtle errors that cause bugs that happen only once every few
hundred or even many thousands of hours.

	And pretty obviously the more control and access the user (eg
applications programmers) get and use the less likely it is that some
combination of separately developed and architected applications and a
particular kernel running on particular hardware will handle all of this
right ALL the time.   Emphasis here on ALL, it usually works most of the
time but making it essentially never fail is really really hard.  And
many of those failures result in things like deadly embrace lockups
which can cause everything to stop or rare conditions in which apparent
causality and temporal coherence completely inverts and things which
"cannot happen" happen exposing all kinds of reasonable but not
quite bulletproof assumptions.

  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."

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