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Exceeding the Complexity Limit in Safe Real-Time Systems



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Virtual Simple Architecture VISA Exceeding the Complexity Limit in Safe Real Time Systems Aravindh Anantaraman Kiran Seth Kaustubh Patil Eric Rotenberg Frank Mueller ECE and CSC Departments Center for Embedded Systems Research North Carolina State Univ avananta krseth ericro ece ncsu edu kspatil mueller cs ncsu edu Abstract Meeting deadlines is a key requirement in safe realtime systems Worst case execution times WCET of tasks are needed for safe planning Contemporary worst case timing analysis tools can safely and tightly bound execution time on in order single issue pipelines with caches and static branch prediction However this simple pipeline appears to be a complexity limit due to the need for analyzability This excludes a whole class of high performance processors from many embedded systems We reconcile the complexity safety trade off by decoupling worst case timing analysis from the processor implementation through a virtual simple architecture VISA A VISA is the timing specification of a hypothetical simple pipeline and is the basis for worst case timing analysis However the underlying microarchitecture can be arbitrarily complex A task is divided into multiple sub tasks which provide a means to gauge progress on the complex pipeline Each sub task is assigned an interim deadline or checkpoint based on the latest allowable completion time of the sub task on the hypothetical simple pipeline If no checkpoints are missed then the complex pipeline is as timely as the safe pipeline If a checkpoint is missed the pipeline switches to a simple mode of operation that directly implements the VISA so that execution time of unfinished sub tasks is safely bounded The significance of our approach is that we circumvent worst case timing analysis of the complex pipeline by dynamically confirming its behavior is bounded by worst case timing analysis of a simpler proxy pipeline The benefit of using a high performance processor is that tasks finish much sooner than they would



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