Global Scheduling Not Required: Simple, Near-Optimal Multiprocessor Real-Time Scheduling with Semi-Partitioned Reservations

Prior work has identified several optimal algorithms for scheduling independent, implicit-deadline sporadic (or periodic) real-time tasks on identical multiprocessors. These algorithms, however, are subject to high conceptual complexity and typically incur considerable runtime overheads. This paper establishes that, empirically, near-optimal schedulability can also be achieved with a far simpler approach that combines three well-known techniques (reservations, semi-partitioned scheduling, and period transformation) with some novel task-placement heuristics.In large-scale schedulability experiments, the proposed approach is shown to achieve near-optimal hard real-time schedulability (99+% schedulable utilization) across a wide range of processor and task counts. With an implementation in LITMUSRT, the proposed approach is shown to be practical and to incur only low runtime overheads, comparable to a conventional partitioned scheduler. It is further shown that basic slack management techniques can help to avoid more than 50% of all migrations of semi-partitioned reservations if tasks execute on average for less than their provisioned worst-case execution time.Two main conclusions are drawn: pragmatically speaking, global scheduling is not required to support static workloads of independent, implicit-deadline sporadic (or periodic) tasks; and since such simple workloads are well supported, future research on multiprocessor real-time scheduling should consider more challenging workloads (e.g., adaptive workloads, dynamic task arrivals or mode changes, shared resources, precedence constraints, etc.).