The Impact of Optics on HPC System Interconnects

Optical signaling has long been used for telecommunications, where its low-loss signaling capability is needed and the relatively high termination costs can be amortized over long distances. Until recently, Cray has not found it advantageous to use optics in its multiprocessor interconnects. With recent reductions in optical costs and increases in signaling rates, however, the situation has changed, and Cray is currently developing a hybrid electrical/optical interconnect for our “Cascade” system, which will be shipping in 2012. In this position paper, Cray was asked to answer the question “Will cost-effective optics fundamentally change the landscape of networking?” The short answer is yes. By breaking the tight relationship between cable length, cost, and signaling speed, optical signaling technology opens the door to network topologies with much longer links than are feasible with electrical signaling. Cost-effective optics will thus enable a new class of interconnects that use high-radix network topologies to significantly improve performance while reducing cost.