nccl/ext-tuner/example

NCCL Example Tuner Plugin

This example plugin shows a practical example of a CSV file-based tuning approach, allowing selective overrides for tuning parameters based on all tuning inputs without recompiling.

Features

• File-based Configuration: Read tuning parameters from a CSV configuration file
• Size-based Tuning: Specify different configurations based on message size ranges
• Dimension-aware Tuning: Match configurations based on number of nodes and ranks
• Optional Channels Configuration: Set specific channel counts or use -1 to keep NCCL's default
• Environment Variable Support: Specify config file location via NCCL_TUNER_CONFIG_FILE
• Fallback Behavior: Gracefully handles missing config files and invalid entries

Building

make

This will create libnccl-tuner-example.so that can be loaded by NCCL.

Configuration File Format

The configuration file uses CSV (Comma-Separated Values) format with one configuration per line:

collective_type,min_bytes,max_bytes,algorithm,protocol,channels,nNodes,nRanks,numPipeOps,regBuff

Parameters

• collective_type: The collective operation type

  • broadcast, reduce, allgather, reducescatter, allreduce

• min_bytes/max_bytes: The message size range (in bytes) for which this config applies

  • Use 0 for minimum and 4294967295 for maximum (covers all sizes)

• algorithm: The NCCL algorithm to use

  • tree, ring, collnet_direct, collnet_chain, nvls, nvls_tree, pat

• protocol: The NCCL protocol to use

  • ll, ll128, simple

• channels: Number of channels (SMs) to use

  • Use a positive integer to specify exact channel count
  • Use -1 to keep NCCL's default channel selection

• nNodes: Number of nodes to match

  • Use a positive integer to match specific node count
  • Use -1 to match any number of nodes

• nRanks: Number of ranks to match

  • Use a positive integer to match specific rank count
  • Use -1 to match any number of ranks

• numPipeOps: Number of pipeline operations to match (optional)

  • Use a positive integer to match specific pipeline operation count
  • Use -1 to match any number of pipeline operations
  • If omitted, configuration will match any numPipeOps value

• regBuff: Whether user buffer can be registered (optional)

  • Use 0 to match only non-registered buffers
  • Use 1 to match only registered buffers
  • Use -1 to match either registered or non-registered buffers
  • If omitted, configuration will match any regBuff value

Example Configuration

# Single-node, small allreduce: use tree algorithm, registered buffers only
allreduce,0,65536,tree,simple,2,1,-1,-1,1

# 4-node, 32-rank setup: medium allreduce, single pipeline op, non-registered buffers
allreduce,65537,1048576,ring,simple,4,4,32,1,0

# Any topology: large allreduce with LL128, multiple pipeline ops, any buffer type
allreduce,1048577,4294967295,ring,ll128,-1,-1,-1,4,-1

# Single-node broadcast: prefer tree, any pipeOps, registered buffers (backward compatible)
broadcast,0,32768,tree,simple,-1,1,-1

# Multi-node broadcast: optimized for non-registered buffers, single pipeline op
broadcast,32769,4294967295,ring,simple,2,-1,-1,1,0

Comments start with # and empty lines are ignored. The CSV format makes it easy to edit configurations in spreadsheet applications like Excel, Google Sheets, or LibreOffice Calc.

Backward Compatibility

Configurations without the numPipeOps and/or regBuff parameters are fully supported:

• 8 fields: matches any numPipeOps and regBuff values
• 9 fields: matches any regBuff value
• 10 fields: full parameter specification

This ensures existing configuration files continue to work without modification.

Usage

Method 1: Default Config File

Place your configuration in nccl_tuner.conf in the current working directory.

Method 2: Environment Variable

Set the NCCL_TUNER_CONFIG_FILE environment variable to specify the config file path:

export NCCL_TUNER_CONFIG_FILE=/path/to/your/tuner.conf
export LD_LIBRARY_PATH=/path/to/plugin:$LD_LIBRARY_PATH
mpirun -np 4 your_nccl_application

Editing Configuration Files

Generating Configuration Files from Raw Data

A python script to generate valid CSV configs has been provided. Using optimize_config.py.

Spreadsheet Tips:

• Use column headers: collective_type,min_bytes,max_bytes,algorithm,protocol,channels,nNodes,nRanks,numPipeOps,regBuff
• Save as CSV format (not Excel format) for the plugin to read
• Use data validation to prevent typos in algorithm/protocol names

Logging

The plugin uses NCCL's logging system. To see tuner-related messages:

export NCCL_DEBUG=INFO

This will show when configurations are loaded and applied, including the topology information.

For detailed debugging output during tuning decisions:

export NCCL_DEBUG=TRACE

This will show verbose information about which configurations are being evaluated and matched.

Dimension Matching

Configurations are only applied when the topology matches:

• Exact Match: Configuration specifies nNodes=4,nRanks=32, only applied when communicator has exactly 4 nodes and 32 ranks
• Wildcard Nodes: Configuration specifies nNodes=-1,nRanks=8, applied to any topology with exactly 8 ranks
• Wildcard Ranks: Configuration specifies nNodes=2,nRanks=-1, applied to any 2-node topology regardless of ranks per node
• Wildcard Both: Configuration specifies nNodes=-1,nRanks=-1, applied to any topology

This allows you to create specialized configurations for different cluster setups while maintaining flexibility.

Default Behavior

If no configuration file is found or no matching configuration exists for a collective operation, the plugin falls back to preferring the ring algorithm with simple protocol. All configured algorithm/protocol combinations are given a low cost (0.0) to make them preferred by NCCL's selection logic.

When channels is set to -1, NCCL's default channel selection logic is preserved, allowing the system to automatically determine the optimal number of channels based on hardware and message size.

Troubleshooting

1. Config file not found: Check the file path and permissions
2. Configurations not applied: Verify the collective type, size ranges, algorithm/protocol names, and topology parameters
3. Plugin not loaded: Ensure LD_LIBRARY_PATH includes the plugin directory
4. No effect on performance: Check that NCCL is actually using the tuner plugin with NCCL_DEBUG=INFO
5. Topology mismatch: Verify that nNodes and nRanks match your actual setup, or use -1 for wildcards
6. CSV parsing errors: Ensure no spaces after commas, or quote fields containing spaces