COST MODELING OF N-VERSION FAULT-TOLERANT SOFTWARE SYSTEMS FOR LARGE-N

A cost model determines system costs for fault-tolerant software syste ms, The model finds the optimal number of program versions to achieve minimum system cost of the fault-tolerant software techniques: N-versi on Programming, Recovery Block, and Consensus Recovery Block, In this case, all versions, the voter, and the acceptance test have the same r eliability, When the parameters for the versions, acceptance test, and voter in the cost function are all equal, the cost of a 3-version sys tem is always optimal, Cost(CRB) much less than Cost(RB) much less tha n Cost(NVP) for each target reliability differing by as much as two or ders of magnitude in some cases, The cost functions were increasing fu nctions of N. When the parameters are not equal, optimality occurred f or other values of N. This was especially the case when the cost expon ent for version-1 was larger than the exponents for the other versions and the acceptance test or voter, As the values of the cost exponents for the version reliabilities become larger, a smaller difference was required between the version-1 exponent and the others to produce alt ernate optima.