Dependency Graph Approach for Multiprocessor Real-Time Synchronization

Over the years, many multiprocessor locking protocols have been designed and analyzed. However, the performance of these protocols highly depends on how the tasks are partitioned and prioritized, and how the resources are shared locally and globally. This paper answers a few fundamental questions when real-time tasks share resources in multiprocessor systems. We explore the fundamental difficulty of the multiprocessor synchronization problem and show that a very simplified version of this problem is NP-hard in the strong sense regardless of the number of processors and the underlying scheduling paradigm. Therefore, the allowance of preemption or migration does not reduce the computational complexity. On the positive side, we develop a dependency-graph approach that is specifically useful for frame-based real-time tasks, i.e., when all tasks have the same period and release their jobs always at the same time. We present a series of algorithms with speedup factors between 2 and 3 under semi-partitioned scheduling. We further explore methodologies for and tradeoffs between preemptive and non-preemptive scheduling algorithms, and partitioned and semi-partitioned scheduling algorithms. Our approach is extended to periodic tasks under certain conditions.