Tetris: Circuit Scheduling for Rearrangeably Non-Blocking Photonic Interconnects

Abstract

Reconfigurable photonic interconnects are emerging as a promising communication architecture for next-generation distributed computing. Yet, most circuit schedulers are designed around an idealized view of the interconnect as either blocking or strictly non-blocking. Practical scalable designs are often rearrangeably non-blocking (RNB), with connections routed through networks of internal $2\times2$ switches. This changes the scheduling problem fundamentally: establishing a new connection can force existing connections to be rerouted, trigger state changes across multiple internal switches, and impose reconfiguration delay on otherwise unrelated traffic. We present Tetris, a circuit scheduling algorithm for RNB photonic interconnects. Tetris builds on two observations. First, All-to-All demands are often not doubly stochastic, leaving a small number of endpoints as communication bottlenecks. Second, reconfiguration delay can be large enough to change which connection should be scheduled next. Tetris prioritizes bottleneck endpoints using their remaining communication and reconfiguration work, while selecting and routing matchings to preserve ongoing connections whenever possible. Matchings ensure progress, but connections are scheduled independently, allowing completed connections to be replaced without matching-wide barriers and incurring delay only at switches whose states change. Our simulation and hardware-emulation results show that Tetris reduces All-to-All demand completion time by up to $6.6$x over Birkhoff--von Neumann-based scheduling and by $30$% over Sunflow. More broadly, RNB interconnects raise new questions in multi-tenant scheduling and routing for partial reconfiguration, which we discuss at the end of the paper.