To be silent or not: on the impact of evictions of clean data in cache-coherent multicores

Abstract

Maintaining coherence across hundreds or even thousands of cores is not an easy task. Among all of the proposed solutions until now, directory-based cache coherence has been advocated as the most feasible way of beating the scalability hurdles that arise at such large scale. Thanks to the knowledge accumulated during the last four decades, there is general consensus on the impact of most of the design aspects of directory coherence on performance, energy consumption and cost. However, there is one subtle design point for which we have observed some divergences in contemporary research works on cache-coherent multicores. Specifically, while some recent works assume a silent replacement policy for evictions of clean data in the last-level private caches, others implement just the opposite that we call a noisy replacement policy, and even others do not mention how these evictions are managed. In this work, we put this important aspect into the spotlight, demonstrating that the way in which evictions of clean data are managed can have important influence on the performance and energy consumption of a directory-based cache coherence protocol. We show that the noisy replacement policy leads to a significant increase in the total traffic (around 20% in several cases, 9.6% on average) compared with the silent policy. Given the important fraction of the total power budget that the on-chip interconnection network of future manycores is expected to consume, assuming the silent replacement policy for clean data will lead to non-negligible energy savings. Moreover, and what is more important, we have observed that depending on the particular directory structure used, assuming silent replacements could affect performance or not. This means that the use of noisy replacements is not justified in all cases, since it would increase unnecessarily network traffic without leading to any performance advantages.