Processor Coupling: Integrating Compile Time and Runtime Scheduling for Parallelism

The technology to implement a single-chip node composed of 4 high-performance floating-point ALUs will be available by 1995. This paper presents processor coupling, a mechanism for controlling multiple ALUs to exploit both instruction-level and inter- thread parallelism, by using compile time and runtime scheduling. The compiler statically schedules individual threads to discover available inter-thread instruction-level parallelism. The runtime scheduling mechanism interleaves threads, exploiting inter-thread parallelism to maintain high ALU utilization. ALUs are assigned to threads on a cycle by cycle basis, and several threads can be active concurrently. We provide simulation results demonstrating that, on four simple numerical benchmarks, processor coupling achieves better performance than purely statically scheduled or multi-processor machine organizations. We examine how performance is affected by restricted communication between ALUs and by long memory latencies. We also present an implementation and feasibility study of a processor coupled node.