Towards Energy-Proportional Optical Interconnects

Photonic interconnects have emerged as the prime candidate technology for efficient networks on chip (NoCs). However, the high optical loss of nanophotonic components coupled with the low efficiency of laser sources result in exceedingly high laser power requirements. As optical interconnects stay on even during periods of system inactivity, most of this power is wasted. In this paper we propose ProLaser, a laser control mechanism that has been co-designed with the cache coherence protocol to turn the lasers off when not needed and turn them back on just-in-time when communication is imminent. Overall, ProLaser saves 49-88% of the laser power and closely tracks (within 2-6%) a perfect prediction scheme with full knowledge of future interconnect requests. Moreover, the power savings of ProLaser allow the cores to exploit a higher power budget and run faster, achieving speedups of 1.5-1.7x and 35-52% lower energy consumption.