safely exploiting multithreaded processors to tolerate memory latency in real-time systems
A coarse-grain multithreaded processor can effectively hide long memory latencies by quickly switching to an alternate task when the active task issues a memory request, improving overall throughput. However, dynamic switching cannot be safely exploited to improve throughput in hard-real-time embedded systems. The schedulability of a task-set (guaranteeing all tasks meet deadlines) must be determined a priori using offline schedulability tests. Any computation/memory overlap must be statically accounted for. We develop a novel analytical framework that bounds the overlap between computation of a pipeline-resident-task and on-going memory transfers of other tasks. A simple closed-form schedulability test is derived, that only depends on the aggregate computation (C) and memory (M) components of tasks. Namely, the technique does not require specificity regarding the location of memory transfers within and among tasks and avoids searching all task permutations for a specific feasible schedule. To the best of our knowledge, this is the first work to provide the necessary formalism for safely and tractably exploiting coarse-grain multithreaded processors to tolerate memory latency in hard-real-time systems, exceeding the schedulability limits of classic real-time theory for uniprocessors. Our techniques make it possible to capitalize on higher frequency embedded processors, despite the widening processor-memory speed gap. Experiments with task-sets from C-lab benchmarks reveal improvement in the schedulability of task-sets, measured as the ability to schedule previously infeasible task-sets or reduce utilization for already feasible task-sets. We also demonstrate proof-of-concept by deploying our method in a cycle-level simulator of an ARM11-like embedded microprocessor augmented with multiple register contexts, the same hardware multithreading support available in Ubicom's IP3023 embedded microprocessor.
multithreading memory latency real-time systems schedulability test worst-case execution time