Priority in SPoC

[Adrian Lawrence <adrian.lawrence@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>]

 Denis A Nicole wrote:
 
 > Anyway, the real reason for the message was to discuss SPoC's
 > implementation of PRI.  The SPoC scheduler essentially uses coroutines; it
 > is not preemptive.  When it gains control, which includes every
 > communication and PAR, it uses two queues, high and low priority, as in
 > the transputer. The high priority queue is always run if possible.
 > Without external communication, a high priority process can _only_ become
 > ready at a scheduling point, so this model ensures that there is never a
 > high priority process waiting on a low priority one.  The scheduler has no
 > preemption, so this guarantee cannot be provided for _external_
 > communications triggering a high priority process.
 
{Impressed}
 
Dennis,

Have you any comment to make on how expensive it would be to detect
priority conflicts? As I have said several times, I suspect that it
would add an unacceptable overhead for software implementations. But you
are in a far better position to know.

Also I wonder whether you have any gut-feeling about whether in
principle SPoC should signal "critical error: priority conflict" on our
little test programs. On safety grounds.

Adrian
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