Scalable cache memory design for large-scale SMT architectures

Abstract

The cache hierarchy design in existing SMT and superscalar processors is optimized for latency, but not for bandwidth. The size of the L1 data cache did not scale over the past decade. Instead, larger unified L2 and L3 caches were introduced. This cache hierarchy has a high overhead due to the principle of containment. It also has a complex design to maintain cache coherence across all levels. Furthermore, this cache hierarchy is not suitable for future large-scale SMT processors, which will demand high bandwidth instruction and data caches with a large number of ports.This paper suggests the elimination of the cache hierarchy and replacing it with one-level caches for instruction and data. Multiple instruction caches can be used in parallel to scale the instruction fetch bandwidth and the overall cache capacity. A one-level data cache can be split into a number of block-interleaved cache banks to serve multiple memory requests in parallel. An interconnect is used to connect the data cache ports to the different cache banks, thus increasing the data cache access time. This paper shows that largescale SMTs can tolerate long data cache hit times. It also shows that small line buffers can enhance the performance and reduce the required number of ports to the banked data cache memory.