Combining hardware and software cache coherence strategies

Efficiently maintaining cache coherence is a major problem in large-scale shared memory multiprocessors. Hardware directory schemes have very high memory requirements, while software-directed schemes must rely on imprecise compile-time memory disambiguation. Recently proposed dynamic directory schemes allocate pointers to blocks only as they are referenced, which significantly reduces their memory requirements, but they still allocate pointers to blocks that do not need them. We show how compiler marking can further reduce the directory size by allocating pointers only when necessary. Using trace-driven simulations, we find that the performance of this new approach is comparable to other coherence schemes, but with significantly lower memory requirements.