GREYHOUND: Hunting Fail-Slows in Hybrid-Parallel Training at Scale

Fail-slows, or stragglers, are common problems in large-scale hybrid-parallel training that runs on a large fleet of GPU servers for an extended period of time. Yet, these problems are not well studied. In this paper, we first present a characterization study on a shared production cluster with over 10,000 GPUs. We find that fail-slows manifest as transient stragglers caused by slow computations or communications due to contention, device degradation, or network congestion, lasting from sub-minutes to nearly ten hours, and delaying large training jobs by 1.34× on average. The current practice is to manually detect fail-slows and treat them as fail-stops by means of checkpoint-and-restart failover, which is time-consuming. In this paper, we propose GREYHOUND, a system that rapidly identifies slow GPUs and/or communication links, and effectively tackles them with a novel multi-level mitigation mechanism, all without human intervention. GREYHOUND correctly detects fail-slows in a production cluster with over 99% accuracy. Testbed experiment on 256 H800 GPUs further shows it effectively handles (manually injected) stragglers, improving end-to-end throughput by 1.58×.