Roger Racine wrote:

> At 04:24 PM 8/9/1999 , Harbaugh, John S wrote:
> >I'm always curious why someone would choose time-slicing as a dispatching
> or scheduling mechanism.  It is a poor-man's  attempt to simulate
> parallelism on a single CPU computer.  Most authors (Ben-Ari, Burns &
> Wellings, to mention a few) recommend that programs be designed around the
> idea of concurrency, the _potential_ for simultaneous execution of multiple
> instructions, rather than parallelism, the required use of multiple
> processors.  A correct, concurrent Ada program using either preemption or
> rendezvous can execute on one or many processors.  Contrast this with a
> parallel program that must rely on time slicing to run correctly on a
> monoprocessor machine.  Would it not be preferable to use basic language
> features rather than rely on non-portable implementation policies?
> >
>
> Time slicing is typically used on general-purpose operating systems to
> provide some equality among processes on the use of CPU.

A general-purpose operating system is usually in effect a time sharing system.
In such a system issues such as fairness and minimizing _average_ response time
are of concern.  You typically have a lot of processes and you don't know much
about them.  A real-time system is usually quite different.  The major issues
are  such things as predictability and schedulability.  You have (relatively)
few tasks and you know a lot about them.  Instead of fairness, you want the
highest priority task to have first call on all system resources it needs to
get the job done.

If you compare a typical host (development) environment with a typical target
(execute only) environment such as a workstation and an imbedded computer
controlling an oil refinery, I think it can be seen that time slicing is
appropriate for the first but in most cases not for the second.

Regards,
Bob
--
                               Robert L. Spooner
                         Registered Professional Engineer
                               Research Assistant
                         Intelligent Control Systems Group

            Applied Research Laboratory        Phone: (814) 863-4120
            The Pennsylvania State University  FAX:   (814) 863-7841
            P. O. Box 30
            State College, PA 16804-0030       [log in to unmask]