Boosting performance of directory-based cache coherence protocols with coherence bypass at subpage granularity and a novel on-chip page table

Abstract

Chip multiprocessors (CMPs) require effective cache coherence protocols as well as fast virtual-to-physical address translation mechanisms for high performance. Directory-based cache coherence protocols are the state-of-the-art approaches in many-core CMPs to keep the data blocks coherent at the last level private caches. However, the area overhead and high associativity requirement of the directory structures may not scale well with increasingly higher number of cores. As shown in some prior studies, a significant percentage of data blocks are accessed by only one core, therefore, it is not necessary to keep track of these in the directory structure. In this study, we have two major contributions. First, we show that compared to the classification of cache blocks at page granularity as done in some previous studies, data block classification at subpage level helps to detect considerably more private data blocks. Consequently, it reduces the percentage of blocks required to be tracked in the directory significantly compared to similar page level classification approaches. This, in turn, enables smaller directory caches with lower associativity to be used in CMPs without hurting performance, thereby helping the directory structure to scale gracefully with the increasing number of cores. Memory block classification at subpage level, however, may increase the frequency of the Operating System's (OS) involvement in updating the maintenance bits belonging to subpages stored in page table entries, nullifying some portion of performance benefits of subpage level data classification. To overcome this, we propose a distributed on-chip page table as a our second contribution.