Symphony: Taming Step Misalignments in the Network for Ring-based Collective Operations

Ring-based collective operations are widely used in distributed AI training due to their efficient bandwidth utilization. While ring communication excels at pipelining, its performance is heavily dependent on having synchronized step-wise progression. This presents a mismatch to the underlying network conditions in practice: collective operations are vulnerable to network jitter and congestion, leading to step misalignment and increased collective completion time. To that end, we propose Symphony, an in-network solution that detects pipeline step misalignment and mitigates its impact. Symphony introduces (1) a lightweight mechanism to track per-job pipeline progress and (2) a novel use of congestion signals to selectively throttle outpacing flows, allowing lagging flows to catch up without global coordination. Through simulations using Astra-Sim, we show that Symphony effectively mitigates step misalignments in ring-based collectives, resulting in up to 54% improvement in job/collective communication time. Finally, we prototype and validate Symphony on an Intel Tofino2 programmable switch to demonstrate its practicality.