Performance Characterization of a Multithreaded Architecture: Where Are the Beneets?

Multithreaded architectures hold the promise of high performance through an overlap of computation and communication. This paper explores how the overlap in multithreaded execution aaects the performance of processor, memory, and network subsystems; what are the critical parameters to ensure high processor performance; and what is the performance impact of optimizations of the workload and architectures. We use McGill's EARTH system as a case study. We conduct empirical measurements to show the real costs of multithreading, and evaluate the performance of multithreaded architectures using analytical models. Our detailed characterization of multithreaded execution includes new program workload parameters like the number of remote (and local) accesses per thread. We use synthetic benchmarks to explore how the program workload parameters aaect the performance individually and in combinations. A small number of remote requests per thread and a high thread runlength are crucial to achieve a high processor performance. While multithreading is expected to tolerate long latencies for remote data accesses, the latencies increase up to 200% higher than their no-load values around 300 cycles, for ne-grain parallel program workload on the EARTH system. Further, a superscalar program execution reduces the computation time, thereby injecting messages more frequently and increasing the latencies. Consequently, the change in processor performance may be little. We identify the delays at the EARTH node as the performance bottlenecks, and explore two EARTH conngurations which can yield higher gains. While the current EARTH implementation can tolerate the long latencies of a NOW system, reducing the delays at an EARTH node allows higher performance at ner granularities. With a performance characterization such as ours, a compiler can choose a right mix of parameter values to achieve the desired performance. Similarly, an architect can evaluate the trade-oos of realistic design implementations.