Virtual simple architecture (VISA)

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 have on an explicitly-safe processor. The new slack in the schedule can be exploited for higher throughput or lower power. With the VISA approach, an arbitrarily complex SMT processor can safely run non-real-time tasks at the same time as a real-time task. Alternatively, frequency/voltage can be safely lowered to take up slack. We explore the latter application and show a VISA-compliant complex pipeline consumes 43--61% less power than an explicitly-safe pipeline.