Optical Overlay NUCA

Abstract

In this article, we propose using optical networks-on-chip (NoCs) to design cache access protocols for large shared L2 caches. We observe that the problem is unique because optical networks have very low latency, and in principle all of the cache banks are very close to each other. A naive approach is to broadcast a request to a set of banks that might possibly contain the copy of a block. However, this approach is wasteful in terms of energy and bandwidth. Hence, we propose a set of novel schemes that create a set of virtual networks ( overlays ) of cache banks over a physical optical NoC. We search for a block inside each overlay using a combination of multicast and unicast messages. We first propose two simple protocols: TSI and Broadcast . The former uses unicast messages, and the latter uses multicast messages. We subsequently propose an improved scheme, OP_BCAST , that combines the best of TSI and Broadcast , and mainly uses restricted multicast messages. Then we propose a set of novel hardware structures for creating and managing overlays, for efficiently locating blocks in the overlay, and for implementing dynamically changing overlays with OP_BCAST . The performance of the TSI scheme is within 2% to 3% of a broadcast scheme, and it is faster than traditional schemes with electrical networks by 26%. Compared to the broadcast scheme, it reduces the number of accesses, and consequently the dynamic energy of the caches by 6% to 8%. OP_BCAST is 34% faster than the best solutions with copper-based NoCs; moreover, it reduces the dynamic energy for cache access by 33% compared to the TSI scheme.