Managing Laser Power in Silicon-Photonic NoC Through Cache and NoC Reconfiguration

In manycore systems, the silicon-photonic link technology is projected to replace electrical link technology for global communication in network-on-chip (NoC) as it can provide as much as an order of magnitude higher bandwidth density and lower data-dependent power. However, a large amount of fixed power is dissipated in the laser sources required to drive these silicon-photonic links, which negates any bandwidth density advantages. This large laser power dissipation depends on the number of on-chip silicon-photonic links, the bandwidth of each link, and the photonic losses along each link. In this paper, we propose to reduce the laser power dissipation at runtime by dynamically activating/deactivating L2 cache banks and switching ON/OFF the corresponding silicon-photonic links in the NoC. This method effectively throttles the total on-chip NoC bandwidth at runtime according to the memory access features of the applications running on the manycore system. Full-system simulation utilizing Princeton application repository for shared-memory computers and Stanford parallel applications for shared-memory-2 parallel benchmarks reveal that our proposed technique achieves on an average 23.8% (peak value 74.3%) savings in laser power, and 9.2% (peak value 26.9%) lower energy-delay product for the whole system at the cost of 0.65% loss (peak value 2.6%) in instructions per cycle on average when compared to the cases where all L2 cache banks are always active.