Fault Tolerant Computer Architecture-P6: For many years, most computer architects have pursued one primary goal: performance. Architects have translated the ever-increasing abundance of ever-faster transistors provided by Moore’s law into remarkable increases in performance. Recently, however, the bounty provided by Moore’s law has been accompanied by several challenges that have arisen as devices have become smaller, including a decrease in dependability due to physical faults | ERROR DETECTION 39 The checker core is so simple that it can be formally verified to be bug-free so no design bugs cause errors in it. The checker is only 6 of the area of an Alpha 21264 core 91 and the performance impact of DIVA is minimal. Comparing DIVA to Argus DIVA achieves slightly better error detection coverage. However DIVA is far more costly when applied to small simple cores instead of superscalar cores because the checker core becomes similar in size to the core it is checking. Watchdog Processors. Most of the invariant checkers we have discussed so far have been tightly integrated into the core. An alternative implementation is a watchdog processor as proposed by Mahmood and McCluskey 42 . A watchdog processor is a simple coprocessor that watches the behavior of the main processor and detects violations of invariants. As illustrated in Figure a typical watchdog shares the memory bus with the main processor. The invariants checked by the watchdog can be any of the ones discussed in this section and the original seminal work by Mahmood and McCluskey checked many invariants including control flow and memory access invariants. High-Level Anomaly Detection The end-to-end argument 64 which we discussed in Section motivates the idea of detecting errors by detecting when they cause higher-level behaviors that are anomalous. In this section we present anomaly detection techniques and we present them from the lowest-level behavioral anomalies to the highest. Data Value Anomalies. The value of a given datum often remains constant or within a narrow range of values during the execution of a program and an aberration from this usual behavior is likely to indicate an error. The expected range of values can be obtained either by statically profiling the program s behavior or by dynamically profiling it at runtime and inferring that this behavior is likely to continue. For example dynamic behavior might reveal that a certain integer is always less .
