H$^2$OEIN: A Hierarchical Hybrid Optical/Electrical Interconnection Network for Exascale Computing Systems

The performance of high-performance computing (HPC) systems is largely determined by the interconnection network. The rising demand for computing capability leads to an expansion of the interconnection network and a corresponding increase in system cost and power consumption. The growing use of optical interconnects not only reduces the network cost and power consumption, but also meets the system-scaling bandwidth demands. However, unlike in an electrical switch, the lack of a buffer in the optical switch makes it hard to operate an all-optical network at packet-level granularity. In this paper, we propose a hierarchical hybrid optical/electrical interconnection network (H $^2$ OEIN) based on low-radix switches and arrayed waveguide grating routers (AWGRs). In the lower layers, the use of low-radix switches results in lower cost and power consumption. The modular structure composed of low-radix switches facilitates the expansion of the network. At higher layers, high bandwidth and fast switching can be achieved using AWGR based optical interconnects. Because the higher layers of the network are passive, the power consumption can be reduced to a large extent. Network simulation results show that H $^2$ OEIN reduces the cost by 25 percent and the power consumption by 45 percent compared to a dragonfly network in configurations with over 300,000 nodes.