A combined performance and reliability (performability) measure for gracefully degradable fault-tolerant systems is introduced and a closed-form, analytic solution is provided for computing the performability of a class of unrepairable systems which can be modeled by general acyclic Markov processes. This allows the study of models which consider the degradation of more than one type of system component, e.g. processors, memories, buses. An efficient evaluation algorithm is provided, with an extensive analysis of its time and space complexity. A numerical example is provided which shows how the combined performance/reliability measure provides for a complete evaluation of the relative merits of different multiprocessor structures.
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