NVIDIA Collective Communications Library (NCCL)

NCCL, which stands for NVIDIA Collective Communications Library, is a library developed by NVIDIA to facilitate efficient communication between multiple GPUs, both within a single node and across multiple nodes in a distributed system.

It is specifically designed to optimise collective communication operations commonly used in deep learning and high-performance computing applications.

Here are the key points to understand about NCCL

Purpose

NCCL aims to provide fast and efficient communication primitives for data exchange between GPUs. It is particularly useful in scenarios where multiple GPUs need to work together to perform computations, such as in distributed deep learning training.

Collective Operations

NCCL supports various collective communication operations, including:

• AllReduce: Reduces data across all GPUs and distributes the result back to all GPUs.

• Broadcast: Sends data from one GPU to all other GPUs.

• Reduce: Reduces data across all GPUs and sends the result to a specified GPU.

• AllGather: Gathers data from all GPUs and distributes the combined data to all GPUs.

• ReduceScatter: Reduces data across all GPUs and scatters the result evenly among the GPUs.

Optimised Performance

NCCL is highly optimised for NVIDIA GPUs and takes advantage of the underlying hardware capabilities, such as NVIDIA NVLink and InfiniBand, to achieve high bandwidth and low latency communication.

It automatically detects the optimal communication paths and algorithms based on the system topology.

Easy Integration

NCCL provides a simple and intuitive API that closely follows the popular Message Passing Interface (MPI) standard.

This makes it easy for developers familiar with MPI to adopt NCCL in their applications. NCCL can be integrated into existing code bases, and it supports various programming models, including single-threaded, multi-threaded, and multi-process (e.g., using MPI).

Compatibility

NCCL is compatible with a wide range of NVIDIA GPUs and can be used across different GPU architectures.

It supports communication within a single node using PCIe and NVLink interconnects, as well as across multiple nodes using high-speed network fabrics like InfiniBand.

Deep Learning Frameworks

Many popular deep learning frameworks, such as TensorFlow, PyTorch, and MXNet, have integrated NCCL to accelerate distributed training on multi-GPU systems.

NCCL enables efficient synchronisation and communication between GPUs, allowing for faster training times and improved scalability.

In summary, NCCL is a powerful library that simplifies and optimises communication between multiple GPUs in a system.

It provides a set of collective communication operations that are essential for distributed computing and deep learning.

By leveraging NCCL, developers can harness the full potential of multi-GPU systems and achieve significant performance improvements in their applications.

NCCL abstracts away the complexities of low-level communication protocols and provides a high-level API that is easy to use and integrate into existing code bases.

It has become a critical component in the ecosystem of GPU-accelerated computing, enabling researchers and practitioners to efficiently scale their workloads across multiple GPUs and nodes.