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

The timing performance of N-version multi-stage software is analyzed for a strategy called expedient voting. In expedient voting the voting takes place as soon as an adequate number of components have finished the 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 three stages offer diminishing returns. Speed-up deteriorates with reduction in the version reliability and independence