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.

NUCA caches have traditionally been proposed as a solution for mitigating wire delays, and delays introduced due to complex networks on chip. Traditional approaches have reported significant performance gains with intelligent block placement, location, replication, and migration schemes. In this paper, we propose a novel approach in this space, called FP-NUCA. It differs from conventional approaches, and relies on a novel method of co-designing the last level cache and the network on chip. We artificially constrain the communication pattern in the NUCA cache such that all the messages travel along a few predefined paths (fast paths) for each set of banks. We leverage this communication pattern by designing a new type of NOC router called the Freeze router, which augments a regular router by adding a layer of circuitry that gates the clock of the regular router when there is a fast path message waiting to be transmitted. Messages along the fast path do not require buffering, switching, or routing. We incorporate a bank predictor with our novel NOC for reducing the number of messages, and resultant energy consumption. We compare our performance with state of the art protocols, and report speedups of up to 31 percent (mean: 6.3 percent), and ED2 reduction up to 46 percent (mean: 10.4 percent) for a suite of Splash and Parsec benchmarks. We implement the Freeze router in VHDL and show that the additional fast path logic has minimal area and timing overheads.

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%.