A Practical Shared Optical Cache With Hybrid MWSR/R-SWMR NoC for Multicore Processors

Abstract

Conventional electronic memory hierarchies are intrinsically limited in their ability to overcome the memory wall due to scaling constraints. Optical caches and interconnects can mitigate these constraints, and enable processors to reach performance and energy efficiency unattainable by purely electronic means. However, the promised benefits cannot be realized through a simple replacement process; to reach its full potential, the architecture needs to be holistically redesigned. This article proposes Pho$, an opto-electronic memory hierarchy architecture for multicores. Pho$ replaces conventional core-private electronic caches with a large shared optical L1 built with optical SRAMs. The shared optical cache is supported by Pho$Net, a novel hybrid MWSR/R-SWMR optical NoC that provides low-latency and high-bandwidth communication between the electronic cores and the shared optical L1 at low optical loss. Pho$Net's unique network arbitration protocol seamlessly co-arbitrates the request and reply sub-networks and facilitates cache requests and replies that optimize for the common case of cache hits. Through Pho$ we solve the problems that render previous designs impractical. Our results show that Pho$ achieves on average 1.41× performance speedup (3.89× max) and 31% lower energy-delay product (90% max) against conventional designs. Moreover, the Pho$Net optical NoC for core-cache communication consumes 70% less power compared to directly applying previously proposed optical NoC architectures.