Optimal configuration of redundant real-time systems in the face of correlated failure

Real-time computers are frequently used in harsh environments, such as space or industry. Lightning strikes, streams of elementary particles, and other manifestations of a harsh operating environment can cause transient failures in processors. Since the entire system is in the same environment, an especially severe disturbance can result in a momentary, correlated, failure of all the processors. To have the system survive transient correlated failures and still execute all its critical workload on time, designers must use time redundancy. To survive permanent or transient independently-occurring failures, processor redundancy must be used, and the computer configured into redundant clusters. Given a fixed total number of processors, there is a tradeoff between processor- and time-redundancy, This paper considers the tradeoffs between configuring the system into duplexes and triplexes. There are pessimistic and optimistic reliability models for each configuration. For the range of pertinent parameters, these models are very close, indicating that these models are quite accurate. The duplex-tripler tradeoff is between the effects of permanent, independent-transient, and correlated-transient failures. Configuring the system in triplexes provides better protection against permanent and independent-transient failures, but diminishes protection against correlated-transient failures. The better configuration is given for each application.