The Big Send-off: Scalable and Performant Collectives for Deep Learning

Collective communication is becoming increasingly important in data center and supercomputer workloads with an increase in distributed AI related jobs. However, existing libraries that provide collective support such as NCCL, RCCL, and Cray-MPICH exhibit several performance and scalability limitations on modern GPU supercomputers. To address these challenges, we introduce the Performant Collective Communication Library (PCCL), specifically targeted for distributed deep learning (DL) workloads. PCCL provides highly optimized implementations of key collectives used in distributed DL: all-gather, reduce-scatter, and all-reduce. PCCL uses a hierarchical design with learning-based adaptive selection of the best performing algorithms to scale efficiently to thousands of GPUs. It achieves substantial performance speedups over RCCL on 2048 GCDs of Frontier -- up to 168x for reduce-scatter, 33x for all-gather and 10x for all-reduce. More modest but still significant gains up to 5.7x over NCCL are observed on Perlmutter. These gains translate directly to performance improvement of production DL workloads: up to 4.9x speedup over RCCL in DeepSpeed ZeRO-3 training, and up to 2.4x speedup in DDP training.