DCOF - An Arbitration Free Directly Connected Optical Fabric

In this paper we investigate the unique potential of optics to provide a family of arbitration free topologies that are not realizable using conventional electronics. This is accomplished by creating a directly connected fabric of waveguides that can be configured to support everything from a crossbar to fully connected topologies. The large number of waveguides required to create a Directly Connected Optical Fabric (DCOF) can be built by taking advantage of multiple photonic layers connected with photonic vias, allowing the architect to choose the degree of simultaneous communication (a parameter called k) necessary to meet the performance requirements and available power budget. In order to evaluate DCOF we developed a detailed implementation model for three different network instantiations a crossbar similar to Corona, DCOF configured as a crossbar, and DCOF configured as a fully connected network. We analyzed the power consumption and performance of these topologies on a variety of benchmarks, including SPLASH-2 and synthetic traces. Our results demonstrate that the overhead required by arbitration is non-trivial, especially at high loads. Eliminating the need for arbitration, sizing the buffers carefully and retransmitting lost packets when there is contention results in a significant reduction in average packet latency without additional power overhead. We also show that when configured as a crossbar DCOF is the most energy efficient while maintaining excellent performance, and when configured as a fully connected network provides the best performance, but at a potentially prohibitive photonic power cost.