Modeling execution time of multi-stage N-version fault-tolerant software

An N-version system can be subdivided into stages for the purpose of forward error recovery through voting after each stage. In the simplest case at each stage the whole system waits for the slowest version to finish before a vote is taken. A better solution is to use a scheme we call Expedient Voting in which the voting takes place as soon as an adequate number of components have finished in a stage. The concept of a "runahead" is introduced  the faster versions are allowed to run ahead of the rest of the slower versions by one or more stages, with synchronized re-start in the event of a failure. If the versions are highly reliable, inter-version failure dependence is small, and the difference between the fastest and the slowest successful components in each stage is large, then the execution speed-up through Expedient Voting may be substantial. Runaheads exceeding 3 stages offer diminishing returns. Speed-up deteriorates with reduction in the version reliability and independence. The advantages and the limitations of Expedient Voting are discussed.