Istituto di Scienza e Tecnologie dell'Informazione     
Bondavalli A., Chiaradonna S., Di Giandomenico F., Strigini L. A contribution to Dependability Evaluation of Fault-Tolerant, Iterative-Execution Software. Internal note CNUCE-B4-95-041, 1995.
We consider the dependability of fault tolerant software executed iteratively, in process controlapplications. We first recall themodels usually adopted for a evaluating the probability of mission survival (reliability ata certain time) and performability, and show the results obtained by applying thesejmodels to the adaptive scheme for software faultolerance SCOP,Self-Configuring Optimal Programming and to the more popular schemes, recovery blocks and multiple version programming. Then weexplore the cons equences on dependability figures of two characteristics of iterative software: a) system failure must be defined in terms of the behaviour of the software over success1ve rterations, becau se the controlled system can usually tolerate short bursts of errors of the control software the probabilistic correlation between successive executions of the software is an important factor in determining the failure behaviour of the software. Positive correlation is to be expected for various reasons, not least the fact that the input values representing physical variables of the controlled system evolve along a "trajectory" in the input space of the software. We present models accounting for these characteristics and evaluate the effects of different distributions of the correlation between successive executions of the software and the sensitivity of the dependability figures to our model parameters

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