Studying the impact of multicore processor scaling on directory techniques via reuse distance analysis

Researchers have proposed numerous directory techniques to address multicore scalability whose behavior depends on the CPU´s particular configuration, e.g. core count and cache size. As CPUs continue to scale, it is essential to explore the directory´s architecture dependences. However, this is challenging using detailed simulation given the large number of CPU configurations that are possible. This paper proposes to use multicore reuse distance analysis to study coherence directories. We develop a framework to extract the directory access stream from parallel LRU stacks, enabling rapid analysis of the directory´s accesses and contents across both core count and cache size scaling. We also implement our framework in a profiler, and apply it to gain insights into multicore scaling´s impact on the directory. Our profiling results show that directory accesses reduce by 3.5x across data cache size scaling, suggesting techniques that tradeoff access latency for reduced capacity or conflicts become increasingly effective as cache size scales. We also show the portion of on-chip memory devoted to the directory cache can be reduced by 53.3% across data cache size scaling, thus lowering the over-provisioning needed at large cache sizes. Finally, we validate our RD-based directory analyses, and find they are within 13% of cache simulations in terms of access count, on average.