Short-circuiting Rings for Low-Latency AllReduce

Abstract: Efficient collective communication is critical for many distributed ML and HPC applications. In this context, it is widely believed that the Ring algorithm for the AllReduce collective communication operation is optimal only for large messages, while Recursive Doubling is preferable for small ones due to its logarithmic number of steps compared to the linear number for Ring. In this paper, we challenge this long-held assumption and show that the Ring algorithm can remain optimal even for short messages in ring-based GPU-to-GPU topologies, once realistic propagation delays and link capacity constraints are accounted for. We find that the total propagation delay for both Ring and Recursive Doubling essentially sums to the same value, but the latter incurs significantly higher congestion due to longer hop counts, leading to increased completion times. This surprising result motivates our case for in-collective adaptive topologies, particularly in the context of emerging photonic interconnects, which can break through the limitations of static topology designs at the collective communication granularity. We design a \emph{simple and fast} heuristic for circuit-switching that enables Recursive Doubling to exploit dynamically reconfigurable photonic paths, carefully balancing reconfiguration delays, propagation latencies, and link congestion to minimize overall completion time. Our preliminary evaluations, using realistic reconfiguration delays, show that our circuit-switching schedules enable faster completion times for Recursive Doubling, even compared to Ring AllReduce on static ring topologies. We conclude by highlighting key challenges and future research directions for realizing practical, in-collective photonic switching.