Exploring grouped coherence for clustered hierarchical cache

Abstract

The industry trends for processors are toward integrating an increasing number of cores into a single chip. Researchers have to deal with frequent data migration across network-on-chip and the increasing on-chip traffic. The innovation from flat to hierarchy is probably a natural design methodology for scalable systems (Martin et al. in Commun ACM, 55(7):78–89, 2012. doi: 10.1145/2209249.2209269 ). Unfortunately, the alternative of hierarchical directory protocol inevitably leads to on-chip traffic overhead, protocol complexity and access latency. In this paper, we target hierarchical cache coherence protocol to overcome the potentially high cost of maintaining cache coherence in current multicore processors. We propose a novel vertical caching protocol combined with grouped coherence, in which the coherence domain expand on demand. More specifically, its design philosophy is to provide a ‘best-effort’ single-copy delivery which allows the shared data only in the first common shared level. Compared to the previous hierarchical protocol, our proposal is able to achieve the performance improvement of 9.9% in the 16-core system and 13.4% in the 64-core system as well as an on-chip traffic reduction of about 10.8% in the 16-core system and 15.9% in the 64-core system, respectively.