The Coming Wave of Multithreaded Chip Multiprocessors

The performance of microprocessors has increased exponentially for over 35 years. However, process technology challenges, chip power constraints, and difficulty in extracting instruction-level parallelism are conspiring to limit the performance of future individual processors. To address these limits, the computer industry has embraced chip multiprocessing (CMP), predominately in the form of multiple high-performance superscalar processors on the same die. We explore the trade-off between building CMPs from a few high-performance cores or building CMPs from a large number of lower-performance cores and argue that CMPs built from a larger number of lower-performance cores can provide better performance and performance/Watt on many commercial workloads. We examine two multi-threaded CMPs built using a large number of processor cores: Sun’s Niagara and Niagara 2 processors. We also explore the programming issues for CMPs with large number of threads. The programming model for these CMPs is similar to the widely used programming model for symmetric multiprocessors (SMPs), but the greatly reduced costs associated with communication of data through the on-chip shared secondary cache allows for more fine-grain parallelism to be effectively exploited by the CMP. Finally, we present performance comparisons between Sun’s Niagara and more conventional dual-core processors built from large superscalar processor cores. For several key server workloads, Niagara shows significant performance and even more significant performance/Watt advantages over the CMPs built from traditional superscalar processors.