Studying the Impact of Multicore Processor Scaling on Cache Coherence Directories via Reuse Distance Analysis

Abstract

Directories are one key part of a processor's cache coherence hardware, and constitute one of the main bottlenecks in multicore processor scaling, e.g. core count and cache size scaling. Many research effects have tried to improve the scalability of the directory, but most of them only simulate a few architecture configurations. It is important to study the directory's architecture dependency, as the CPUs continue to scale. This is because besides applications, directory behaviors are also highly sensitive to architecture. Varying core count directly affects the amount of sharing in the directory, and varying the data cache hierarchy affects the directory access stream. But unfortunately, exploring the huge design space of multiple core counts and cache configurations is challenging using traditional architectural simulation due to the slow speed of simulations.

This thesis studies the directory using multicore reuse distance analysis. It extends existing multicore reuse distance techniques, developing a method to extract directory access information from the parallel LRU stacks used to acquire private-stack reuse distance profiles. This thesis implements this method in a PIN-based profiler to study the directory behavior, including the directory access pattern and directory content, and to analyze current directory techniques.

The profile results show that the directory accesses are highly dependent on cache size, exhibiting a 3.5x drop when scaling the data cache size from 16KB to 1MB; the sharing causes the ratio of directory entry to cache blocks to drop below 50%; and the majority of the accesses are to a small percentage of the directory entries. Cache simulations are performed to validate the profiling results, showing the profiled results are within 14.5% of simulation on average. This thesis also analyzes different directory techniques using the insights from the profiler. The case studies on the Cuckoo, DGD, SCD techniques and multi-level directories show that required directory size varies significantly with CPU scaling, the opportunity of compressing private data decreases with cache scaling, reducing the sharer list size is an effective technique and a small L1 directory is sufficient to capture most of the latency critical accesses respectively.