[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: Priority revisited: a new primitive
Greetings O occam-volk
>Coming from a hardware implementation angle, I still have difficulty with
>"PRI PAR" which appears to be a complete oxymoron.
>Its behaviour (in the non-preemptive case - not that there is any indication
>of preemptive or not, it's left to the implementation) is:
> SEQ i=0 FOR number of "parallel" processes
> Choose the highest priority process
> Do it and disable it
>Which doesn't look very "PAR" to me!
This is not the condemnation I would make. PRI PAR is expressing queue
semantics - a priority queue, rather than a simple one. The availability
of N processors would merely allow the first N processes in a priority
queue to run simultaneously. Remember, however large N is, it may still
be less than the number of processes. We're covered either way.
I would choose to damn this construct because it introduces a SECOND
prioritization, and one that is only vaguely defined, even at the
intuitive level. At best, it might indicate a prioritization of the first
event of each process. But then we can already prioritize behaviour by
event using PRI ALT.
I believe priority should be associated with events, and events only.
That is both intuitively clear and (potentially) semantically more
tractible. Surely, this is all we need when we design any system - to
enable response to events according to their priority. If all processes
truly run in parallel, there is no problem.
A problem only arises when we account for the possibility where there may
fewer processors than processes. There is then the need for a global
measure of priority. This is how I model what PRI PAR is doing. But it
does it very clumsily. In...
...response to ALL events in A receive a higher effective priority than
that to any in B. What if A is our flight control system and B is our
defence? Think I'd get out and walk!
Over the years, I've missed a lot of the occam discussions due to other
activities and interests. Have any of the real-time volk previously
complained, or proposed better solutions?
We seem to be accumulating a wish-list for a new language...
Dr. Ian Robert East
Room T656, Tonge Building
School of Computing and Mathematical Sciences
Oxford Brookes University
2000/2001 Term 1 Consultation hours:
Tuesday 11.00..12.00; 12.00..13.00
Friday 10.00..11.00; 12.00..13.00