SLaC: Stage laser control for a flattened butterfly network

Photonic interconnects have emerged as a promising candidate technology for high-performance energy-efficient on-chip, on-board, and datacenter-scale interconnects. However, the high optical loss of many nanophotonic components coupled with the low efficiency of current laser sources result in exceedingly high total power requirements for the laser. As optical interconnects stay on even during periods of system inactivity, most of this power is wasted, which has prompted research on laser gating. Unfortunately, prior work on laser gating has only focused on low-scalability on-chip photonic interconnects (photonic crossbars), and disrupts the connectivity of the network which renders a high-performance implementation challenging. In this paper we propose SLaC, a laser gating technique that turns on and off redundant paths in a photonic flattened-butterfly network to save laser energy while maintaining high performance and full connectivity. Maintaining full connectivity removes the laser turn-on latency from the critical path and results in minimal performance degradation. SLaC is equally applicable to on-chip, on-board, and datacenter level interconnects. For on-chip and multi-chip applications, SLaC saves up to 67% of the laser energy (43-57% on average) when running real-world workloads. On a datacenter network, SLaC saves 79% of the laser energy on average when running traffic traces collected from university datacenter servers.