The role of optics in future high radix switch design

For large-scale networks, high-radix switches reduce hop and switch count, which decreases latency and power. The ITRS projections for signal-pin count and per-pin bandwidth are nearly flat over the next decade, so increased radix in electronic switches will come at the cost of less per-port bandwidth. Silicon nanophotonic technology provides a long-term solution to this problem. We first compare the use of photonic I/O against an all-electrical, Cray YARC inspired baseline. We compare the power and performance of switches of radix 64, 100, and 144 in the 45, 32, and 22 nm technology steps. In addition with the greater off-chip bandwidth enabled by photonics, the high power of electrical components inside the switch becomes a problem beyond radix 64. We propose an optical switch architecture that exploits high-speed optical interconnects to build a flat crossbar with multiple-writer, single-reader links. Unlike YARC, which uses small buffers at various stages, the proposed design buffers only at input and output ports. This simplifies the design and enables large buffers, capable of handling ethernet-size packets. To mitigate head-of-line blocking and maximize switch throughput, we use an arbitration scheme that allows each port to make eight requests and use two grants. The bandwidth of the optical crossbar is also doubled to to provide a 2x internal speedup. Since optical interconnects have high static power, we show that it is critical to balance the use of optical and electrical components to get the best energy efficiency. Overall, the adoption of photonic I/O allows 100,000 port networks to be constructed with less than one third the power of equivalent all-electronic networks. A further 50% reduction in power can be achieved by using photonics within the switch components. Our best optical design performs similarly to YARC for small packets while consuming less than half the power, and handles 80% more load for large message traffic.