A Feasibility Study of Hierarchical Multithreading

Many studies have shown that significant amounts of parallelism exist at different granularities. Execution models such as superscalar and VLIW exploit parallelism from a single thread. Multithreaded processors make a step towards exploiting parallelism from different threads, but are not geared to exploit parallelism at different granularities (fine and medium grain). In this paper we present a feasibility study of a new execution model for exploiting both adjacent and distant parallelism in the dynamic instruction stream. Our model, called hierarchical multithreading, uses a two-level hierarchical arrangement of processing elements. The lower level of the hierarchy exploits instruction-level parallelism and fine-grain threadlevel parallelism, whereas the upper level exploits more distant parallelism. Detailed simulation studies with a cycleaccurate simulator are presented, showing the feasibility of hierarchical multithreading. Conclusions are drawn about the best ways to obtain the most from the hierarchical multithreading scheme.