TheRock/rocm-systems/projects/rccl/examples/02_point_to_point/README.md

NCCL Point-to-Point Communication Examples

Overview

This directory contains minimal examples that demonstrate NCCL point-to-point (P2P) communication patterns on a single node. These examples focus on clarity and correct communicator usage, resource management, and verification.

Examples

01_ring_pattern

Ring Communication Pattern

• Pattern: Circular data flow among all GPUs
• API: ncclCommInitAll with P2P operations (ncclSend/ncclRecv)
• Use case: Learning P2P communication; pipeline/data movement patterns on a single node
• Key features:
  • Initializes all GPUs in a single process
  • Computes ring neighbors with modulo arithmetic
  • Uses ncclGroupStart/End to prevent deadlocks
  • Verifies data correctness after transfers

Choosing the Right Pattern

Scenario : Pipeline parallel training needs to send data from one GPU to another Addresses : Individual transfers between two ranks Dependencies : A functional NCCL library and its dependencies

Why ncclCommInitAll here?

For single-node collective examples we use ncclCommInitAll as it creates a clique of communicators in one call.

// Initialize all GPUs in one call
ncclComm_t* comms;
int num_gpus;
NCCLCHECK(ncclCommInitAll(comms, num_gpus, NULL));

Building

Quick Start

# Build example by directory name
make 01_ring_pattern

Individual Examples

# Build and run the ring pattern
cd 01_ring_pattern && make
./ring_pattern

References

• NCCL User Guide: Examples
• NCCL API Reference
• HIP Programming Guide