A multithreaded PowerPC processor for commercial servers

This paper describes the microarchitecture of the RS64 IV, a multithreaded PowerPC® processor, and its memory system. Because this processor is used only in IBM iSeries™ and pSeries™ commercial servers, it is optimized solely for commercial server workloads. Increasing miss rates because of trends in commercial server applications and increasing latency of cache misses because of rapidly increasing clock frequency are having a compounding effect on the portion of execution time that is wasted on cache misses. As a result, several optimizations are included in the processor design to address this problem. The most significant of these is the use of coarse-grained multithreading to enable the processor to perform useful instructions during cache misses. This provides a significant throughput increase while adding less than 5% to the chip area and having very little impact on cycle time. When compared with other performance-improvement techniques, multithreading yields an excell ent ratio of performance gain to implementation cost. Second, the miss rate of the L2 cache is reduced by making it four-way associative. Third, the latency of cache-to-cache movement of data is minimized. Fourth, the size of the L1 caches is relatively large. In addition to addressing cache misses, pipeline “holes” caused by branches are minimized with large instruction buffers, large L1 I-cache fetch bandwidth, and optimized resolution of the branch direction. In part, the branches are resolved quickly because of the short but efficient pipeline. To minimize pipeline holes due to data dependencies, the L1 D-cache access is optimized to yield a one-cycle load-to-use penalty.