Quasi-Static Scheduling of Embedded Software Using Equal Conflict Nets

Embedded system design requires the use of efficient scheduling policies to execute on shared resources, e.g. the processor, algorithms that consist of a set of concurrent tasks with complex mutual dependencies. Scheduling techniques are called static when the schedule is computed at compile time, dynamic when some or all decisions are made at run-time. The choice of the scheduling policy mainly depends on the specification of the system to be designed. For specifications containing only data computation, it is possible to use a fully static scheduling technique, while for specifications containing data-dependent control structures, like the if-then-else or while-do constructs, the dynamic behaviour of the system cannot be completely predicted at compile time and some scheduling decisions are to be made at run-time. For such applications we propose a Quasi-static scheduling (QSS) algorithm that generates a schedule in which run-time decisions are made only for data-dependent control structures. We use Equal Conflict (EC) nets as underlying model, and define quasi-static schedulability for EC nets. We solve QSS by reducing it to a decomposition of the net into conflict-free components. The proposed algorithm is complete, in that it can solve QSS for any EC net that is quasi-statically schedulable.