Software Voting in Asynchronous NMR (N-Modular Redundancy) Computer Structures.

Abstract : This paper has explored some of the attributes of NMR computer systems. Many features of software voters have been explored both experimentally and theoretically. Section 2 has presented some software voting concepts. N-modular redundancy has been described and the software concepts of time skew and space redundancy have been explained. Various synchronization issues have been presented, including time-outs, points of synchronization, and asynchronous versus synchronous systems. The frequency of voting and the data granularity were shown to be important factors in determining the reliability of NMR systems. Finally, a technique was described to allow easy bit-by-bit voting on words of data. In Section 4, some experiments were presented to help measure the overhead involved in software voting. The type of voter, the voting frequency, and the average instruction execution time were incorporated into a model of voting overhead. Section 5 shows a number of synchronization experiments. The amount of variation in process execution speed that can be tolerated was determined for three different types of variation.