ForestColl: Throughput-Optimal Collective Communications on Heterogeneous Network Fabrics

Abstract: As modern DNN models grow ever larger, collective communications between the accelerators (allreduce, etc.) emerge as a significant performance bottleneck. Designing efficient communication schedules is challenging, given today's heterogeneous and diverse network fabrics. We present ForestColl, a tool that generates throughput-optimal schedules for any network topology. ForestColl constructs broadcast/aggregation spanning trees as the communication schedule, achieving theoretical optimality. Its schedule generation runs in strongly polynomial time and is highly scalable. ForestColl supports any network fabrics, including both switching fabrics and direct accelerator connections. We evaluated ForestColl on multi-box AMD MI250 and NVIDIA DGX A100 platforms. ForestColl showed significant improvements over the vendors' own optimized communication libraries, RCCL and NCCL, across various settings and in LLM training. ForestColl also outperformed other state-of-the-art schedule generation techniques with both more efficient generated schedules and substantially faster schedule generation speed.