Optical overlay NUCA: A high speed substrate for shared L2 caches

In this paper, we propose to use optical 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 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 novel scheme in this paper, TSI, which proposes to 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 additionally create support for our overlay networks by proposing optimizations to the previously proposed R-SWMR network. We also propose a set of novel hardware structures for creating and managing overlays, and for efficiently locating blocks in the overlay. The performance of the TSI scheme is within 2-3% of a broadcast scheme, and it is faster than traditional static NUCA schemes by 50%. As compared to the broadcast scheme it reduces the number of accesses, and consequently the dynamic energy by 20-30%.