Re: Real Time Scheduling Methods

[bu.edu!inmet!spock!stt@bloom-beacon.mit.edu (Tucker Taft)]

In article <1992Oct28.203415.9815@news.uiowa.edu> 
  jones@pyrite.cs.uiowa.edu 
  (Douglas W. Jones,201H MLH,3193350740,3193382879) writes:

>From article <92302.34020.LJ10891@LMSC5.IS.LMSC.LOCKHEED.COM>,
>by LJ10891@LMSC5.IS.LMSC.LOCKHEED.COM:
>> 
>>    Sorry about that guys, but it looks like Rate Monotonic is the only game
>> in town.
>
>No, there's deadline based scheduling.  Deadline based scheduling is
>incompatable with Ada, because it requires that the priority of each
>process be, at any instant, the deadline by which the process must
>complete its next critical action.  

Deadline-based scheduling is not actually incompatible with Ada.

In Ada 83, if no priority is specified for any task, then all
scheduling is totally implementation-defined.  An implementor
may provide a separate package to set the Deadline for a task,
thereby supporting an alternative scheduling paradigm.

In Ada 9X, alternative scheduling paradigms are acknowledged
explicitly.  The only requirement is that a compiler conforming
to the Real-Time Annex provide support for strict priority-based
scheduling, though it may support other scheduling algorithms as
well.  Furthermore, priorities can be dynamic in Ada 9X,
so almost any scheduling approach can be mapped to
fiddling at appropriate moments with the priority of some
or all of the tasks (hardly an efficient way to do it,
but perhaps useful to keep the semantic model simple).

> . . .
>Thus, deadline based scheduling can be superior to rate monotonic
>scheduling, but it's illegal in Ada.

Not really, as pointed out above.  Of course, if no Ada compiler
or RTS in existence supports it, then who cares anyway?  But
it is definitely not "illegal" for an ambitious Ada compiler- or
RTS-implementor to try to provide deadline-based scheduling if they
so choose.

>					Doug Jones
>					jones@cs.uiowa.edu

S. Tucker Taft         stt@inmet.com
Ada 9X Mapping/Revision Team
Intermetrics, Inc.
Cambridge, MA  02138