Organizational Results

The investigation was conducted in cooperation with the U. S. Department of Transportation, Federal Highwa

ABSTRACT FLEXIBLE SCHEDULING IN MIDDLEWARE FOR DISTRIBUTED RATE-BASED REAL-TIME APPLICATIONS

Distributed rate-based real-time systems, such as process control and avionics mission computing systems, have traditionally been scheduled statically. Static schedul-ing provides assurance of schedulability prior to run-time and can be implemented with low run-time overhead. However, static scheduling is brittle in the face of unantici-pated overload, and treats invocation-to-invocation variations in resource requirements inflexibly. As a consequence, processing resources are often under-utilized in the aver-age case, and the resulting systems are hard to adapt to meet new real-time processing requirements. Dynamic scheduling offers relief from the limitations of static scheduling. How-ever, dynamic scheduling often has a higher run-time cost because certain decisions are enforced on-line. Furthermore, under conditions of overload tasks can be scheduled dynamically that may never be dispatched, or that upon dispatch would miss their dead-lines. We review the implications of these factors on rate-based distributed systems, and posits the necessity to combine static and dynamic approaches to exploit the strengths and compensate for the weaknesses of either approach in isolation