Comparative Evaluation of Latency-Tolerating and -Reducing Techniques for Hardware-Only and Software-Only Directory Protocols

Abstract

We study in this paper how effective latency-tolerating and -reducing techniques are at cutting the memory access times for shared-memory multiprocessors with directory cache protocols managed by hardware and software. A critical issue for the relative efficiency is how many protocol operations such techniques trigger. This paper presents a framework that makes it possible to reason about the expected relative efficiency of a latency-tolerating or -reducing technique by focusing on whether the technique increases, decreases, or does not change the number of protocol operations at the memory module. Since software-only directory protocols handle these operations in software they will perform relatively worse unless the technique reduces the number of protocol operations. Our experimental results from detailed architectural simulations driven by six applications from the SPLASH-2 parallel program suite confirm this expectation. We find that while prefetching performs relatively worse on software-only directory protocols due to useless prefetches, there are examples of protocol optimizations, e.g., optimizations for migratory data, that do relatively better on software-only directory protocols. Overall, this study shows that latency-tolerating techniques must be more carefully selected for software-centric than for hardware-centric implementations of distributed shared-memory systems.