Using variable-MHz microprocessors to efficiently handle uncertainty in real-time systems

Guaranteed performance is critical in real-time systems because correct operation requires tasks complete on time. Meanwhile, as software complexity increases and deadlines tighten, embedded processors inherit high-performance techniques such as pipelining, caches, and branch prediction. Guaranteeing the performance of complex pipelines is difficult and worst-case analysis often under-estimates the microarchitecture for correctness. Ultimately, the designer must turn to clock frequency as a reliable source of performance. The chosen processor has a higher frequency than is needed most of the time, to compensate for uncertain hardware enhancements-partly defeating their intended purpose. We propose using microarchitecture simulation to produce accurate but not guaranteed-correct worst-case performance bounds. The primary clock frequency is chosen based on simulated-worst-case performance. Since static analysis cannot confirm simulated-worst-case bounds, the microarchitecture is also backed up by clock frequency reserves. When running a task, the processor periodically checks for interim microarchitecture performance failures. These are expected to be rare, but frequency reserves are available to guarantee the final deadline is met in spite of interim failures. Experiments demonstrate significant frequency reductions, e.g., -100 MHz for a peak 300 MHz processor. The more conservative worst-case analysis is, the larger the frequency reduction. The shorter the deadline, the larger the frequency reduction. And reserve frequency is generally no worse than the high frequency produced by conventional worst-case analysis, i.e., the system degrades gracefully in the presence of transient performance faults.