Addressing isolation challenges of non-blocking caches for multicore real-time systems

In multicore real-time systems, cache partitioning is commonly used to achieve isolation among different cores. We show, however, that space isolation achieved by cache partitioning does not necessarily guarantee predictable cache access timing in modern COTS multicore platforms, which use non-blocking caches. We find that special hardware registers in non-blocking caches, known as miss status holding registers, which track the status of outstanding cache-misses, can be a significant source of contention that is not addressed by conventional cache partitioning. We propose a hardware and system software (OS) collaborative approach to efficiently eliminate MSHR contention for multicore real-time systems. Our approach includes a low-cost hardware extension that enables dynamic control of per-core memory-level parallelism (MLP) by the OS. Using the hardware extension, the OS scheduler then globally controls each core’s MLP in such a way that eliminates MSHR contention and maximizes overall throughput of the system. We implement the hardware extension in a cycle-accurate full-system simulator and the scheduler modification in Linux 3.14 kernel. Extensive experimental results demonstrate the significance of the MSHR contention problem and the effectiveness of the proposed solution.