Real time systems are being increasingly used in several applications which are time critical in nature. Fault tolerance is an important requirement of such systems, due to the catastrophic consequences of not tolerating faults. We study a scheme that provides fault tolerance through scheduling in real time multiprocessor systems. We schedule multiple copies of dynamic, aperiodic, nonpreemptive tasks in the system, and use two techniques that we call deallocation and overloading to achieve high acceptance ratio (percentage of arriving tasks scheduled by the system). The paper compares the performance of our scheme with that of other fault tolerant scheduling schemes, and determines how much each of deallocation and overloading affects the acceptance ratio of tasks. The paper also provides a technique that can help real time system designers determine the number of processors required to provide fault tolerance in dynamic systems. Lastly, a formal model is developed for the analysis of systems with uniform tasks.
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