Photonic Interconnects for Exascale and Datacenter Architectures

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Exascale and datacenter systems require terabits per second of internode communication bandwidth to meet the performance demands of high-performance computing applications. High-radix routers combined with scalable dragonfly topology have been proposed to reduce execution time and improve power dissipation. Although the dragonfly network has low diameter for exascale networks, fewer global links reduce the bisection bandwidth and require adaptive routing to prevent hot spots due to congestion. Moreover, the number of ports in a high-radix router affects the router cost when implemented with alternate emerging technologies. In this article, the authors advocate multitier network topologies that combine scalable topologies for local (intracabinet) and global (intercabinet) interconnects such as the k-ary n-cube, the flattened butterfly, and the dragonfly, to lead to improved bisection, manageable radix, and reduced link costs, albeit at higher packet latency owing to increased diameter. Because the performance per watt delivered by metallic interconnects or coaxial cables significantly exceeds the available power budget, we envision an entire exascale network composed of photonic links for communication and CMOS routers for switching. Results indicate that multitier topologies are comparable to the single-level dragonfly topology in terms of power and latency while providing higher bisection and reduced area overhead.