A Fully Preemptive Multiprocessor Semaphore Protocol for Latency-Sensitive Real-Time Applications

Independence preservation, a property in real-time locking protocols that isolates latency-sensitive tasks from delays due to unrelated critical sections, is identified, formalized, and studied in detail. The key to independence preservation is to ensure that tasks remain fully preemptive at all times. For example, on uniprocessors, the classic priority inheritance protocol is independence-preserving. It is shown that, on multiprocessors, independence preservation is impossible if job migrations are disallowed. The O(m) independence-preserving protocol (OMIP), a new, asymptotically optimal binary sempahore protocol based on migratory priority inheritance, is proposed and analyzed. The OMIP is the first independence-preserving, real-time, suspension-based locking protocol for clustered job-level fixed-priority scheduling. It is shown to benefit latency-sensitive workloads, both analytically by means of schedulability experiments, and empirically using response-time measurements in LITMUS^RT.