A Memory Subsystem Consisting of an on - Board Crossbar , Level - 2 Cache , and Memory Controllers for a Highly

0272-1732/05/$20.00 ”2005 IEEE Published by the IEEE computer Society Over the past two decades, microprocessor designers have focused on improving the performance of a single thread in a desktop processing environment by increasing frequencies and exploiting instruction level parallelism (ILP) using techniques such as multiple instruction issue, out-of-order issue, and aggressive branch prediction. The emphasis on single-thread performance has shown diminishing returns because of the limitations in terms of latency to main memory and the inherently low ILP of applications. This has led to an explosion in microprocessor design complexity and made power dissipation a major concern. For these reasons, Sun Microsystems’ Niagara processor takes a radically different approach to microprocessor design. Instead of focusing on the performance of single or dual threads, Sun optimized Niagara for multithreaded performance in a commercial server environment. This approach increases application performance by improving throughput, the total amount of work done across multiple threads of execution. This is especially effective in commercial server applications such as databases and Web services, which tend to have workloads with large amounts of thread level parallelism (TLP). In this article, we present the Niagara processor’s architecture. This is an entirely new implementation of the Sparc V9 architectural specification, which exploits large amounts of on-chip parallelism to provide high throughput. Niagara supports 32 hardware threads by combining ideas from chip multiprocessors and fine-grained multithreading. Other studies have also indicated the significant performance gains possible using this approach on multithreaded workloads. The parallel execution of many threads effectively hides memory latency. However, having 32 threads places a heavy demand on the memory system to support high bandPoonacha Kongetira Kathirgamar Aingaran Kunle Olukotun