Directoryless Shared Memory Coherence using Execution Migration

We introduce the concept of deadlock-free migration-based coherent shared memory to the NUCA family of architec-tures. Migration-based architectures move threads among cores to guarantee sequential semantics in large multicores. Using a execution migration (EM) architecture, we achieve performance comparable to directory-based architectures without using directories: avoiding automatic data repli-cation significantly reduces cache miss rates, while a fast network-level thread migration scheme takes advantage of shared data locality to reduce remote cache accesses that limit traditional NUCA performance. EM area and energy consumption are very competi-tive, and, on the average, it outperforms a directory-based MOESI baseline by 1.3 × and a traditional S-NUCA design by 1.2×. We argue that with EM scaling performance has much lower cost and design complexity than in directory-based coherence and traditional NUCA architectures: by merely scaling network bandwidth from 256 to 512 bit flits, the performance of our architecture improves by an addi-tional 13%, while the baselines show negligible improve-ment