NVIDIA Deep Learning NCCL Documentation
Release Notes (PDF) - v2.21.5 - Last updated April 5, 2024

NCCL Release 2.0.2

Key Features and Enhancements

This NCCL release includes the following key features and enhancements.
  • NCCL 2.0.2 provides support for intra-node and inter-node communication.
  • NCCL optimizes intra-node communication using NVLink, PCI express, and shared memory.
  • Between nodes, NCCL implements fast transfers over sockets or InfiniBand verbs.
  • GPU-to-GPU and GPU-to-Network direct transfers, using the GPU Direct technology, is extensively used when the hardware topology permits it.

Using NCCL 2.0.2

Ensure you are familiar with the following notes when using this release.
  • The NCCL 2.0 API is different from NCCL 1.x. Some porting may be needed for NCCL 1.x applications to work correctly. Refer to the migration documentation in the NCCL Developer Guide.

Known Issues

  • NCCL 2.0.2 is known to not work with CUDA driver 384.40 and later.
  • If NCCL returns any error code, set the environment variable NCCL_DEBUG to WARN to receive an explicit error message.
  • NCCL 2.0.2 does not support RoCE, that is, InfiniBand cards using Ethernet as link layer. The presence of an RoCE card on a node will make NCCL fail even when run within the node.
  • Using multiple processes in conjunction with multiple threads to manage the different GPUs may in some cases cause ncclCommInitRank to fail while establishing IPCs (cudaIpcOpenMemHandle). This problem does not appear when using only processes or only threads.