simpo-training

Teaching a model which of two answers people prefer normally means keeping a second, untouched copy of it alongside as a yardstick; this method throws that copy away and still comes out ahead of the one that keeps it.

WHEN TO USE

SCOPE

USE FOR

Align a model to chosen/rejected preference pairs without loading a separate reference model.
Cut training cost and complexity compared to running DPO or PPO.
Preserve a model's existing capabilities by adding SFT regularization during alignment.
Fix training divergence by lowering the learning rate or the beta hyperparameter.

NOT FOR

Use OpenRLHF instead for multi-node distributed training with PPO or GRPO.
Use TRL instead when you need several alignment methods in one framework.
Use DPO instead as an established baseline for comparison.

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In this document (8)

SimPO - Simple Preference Optimization

Quick start

SimPO is a reference-free preference optimization method that outperforms DPO without needing a reference model.

Installation:

# Create environment
conda create -n simpo python=3.10 && conda activate simpo

# Install PyTorch 2.2.2
# Visit: https://pytorch.org/get-started/locally/

# Install alignment-handbook
git clone https://github.com/huggingface/alignment-handbook.git
cd alignment-handbook
python -m pip install .

# Install Flash Attention 2
python -m pip install flash-attn --no-build-isolation

Training (Mistral 7B):

ACCELERATE_LOG_LEVEL=info accelerate launch \
  --config_file accelerate_configs/deepspeed_zero3.yaml \
  scripts/run_simpo.py \
  training_configs/mistral-7b-base-simpo.yaml

Common workflows

Workflow 1: Train from base model (Mistral 7B)

Config (mistral-7b-base-simpo.yaml):

# Model
model_name_or_path: mistralai/Mistral-7B-v0.1
torch_dtype: bfloat16

# Dataset
dataset_mixer:
  HuggingFaceH4/ultrafeedback_binarized: 1.0
dataset_splits:
  - train_prefs
  - test_prefs

# SimPO hyperparameters
beta: 2.0                  # Reward scaling (2.0-10.0)
gamma_beta_ratio: 0.5       # Target margin (0-1)
loss_type: sigmoid          # sigmoid or hinge
sft_weight: 0.0             # Optional SFT regularization

# Training
learning_rate: 5e-7         # Critical: 3e-7 to 1e-6
num_train_epochs: 1
per_device_train_batch_size: 1
gradient_accumulation_steps: 8

# Output
output_dir: ./outputs/mistral-7b-simpo

Launch training:

accelerate launch --config_file accelerate_configs/deepspeed_zero3.yaml \
  scripts/run_simpo.py training_configs/mistral-7b-base-simpo.yaml

Workflow 2: Fine-tune instruct model (Llama 3 8B)

Config (llama3-8b-instruct-simpo.yaml):

model_name_or_path: meta-llama/Meta-Llama-3-8B-Instruct

dataset_mixer:
  argilla/ultrafeedback-binarized-preferences-cleaned: 1.0

beta: 2.5
gamma_beta_ratio: 0.5
learning_rate: 5e-7
sft_weight: 0.1             # Add SFT loss to preserve capabilities

num_train_epochs: 1
per_device_train_batch_size: 2
gradient_accumulation_steps: 4
output_dir: ./outputs/llama3-8b-simpo

Launch:

accelerate launch --config_file accelerate_configs/deepspeed_zero3.yaml \
  scripts/run_simpo.py training_configs/llama3-8b-instruct-simpo.yaml

Workflow 3: Reasoning-intensive tasks (lower LR)

For math/code tasks:

model_name_or_path: deepseek-ai/deepseek-math-7b-base

dataset_mixer:
  argilla/distilabel-math-preference-dpo: 1.0

beta: 5.0                   # Higher for stronger signal
gamma_beta_ratio: 0.7       # Larger margin
learning_rate: 3e-7         # Lower LR for reasoning
sft_weight: 0.0

num_train_epochs: 1
per_device_train_batch_size: 1
gradient_accumulation_steps: 16

When to use vs alternatives

Use SimPO when:

Want simpler training than DPO (no reference model)
Have preference data (chosen/rejected pairs)
Need better performance than DPO
Limited compute resources
Single-node training sufficient

Algorithm selection:

SimPO: Simplest, best performance, no reference model
DPO: Need reference model baseline, more conservative
PPO: Maximum control, need reward model, complex setup
GRPO: Memory-efficient RL, no critic

Use alternatives instead:

OpenRLHF: Multi-node distributed training, PPO/GRPO
TRL: Need multiple methods in one framework
DPO: Established baseline comparison

Common issues

Issue: Loss divergence

Reduce learning rate:

learning_rate: 3e-7  # Reduce from 5e-7

Reduce beta:

beta: 1.0  # Reduce from 2.0

Issue: Model forgets capabilities

Add SFT regularization:

sft_weight: 0.1  # Add SFT loss component

Issue: Poor preference separation

Increase beta and margin:

beta: 5.0            # Increase from 2.0
gamma_beta_ratio: 0.8  # Increase from 0.5

Issue: OOM during training

Reduce batch size:

per_device_train_batch_size: 1
gradient_accumulation_steps: 16  # Maintain effective batch

Enable gradient checkpointing:

gradient_checkpointing: true

Advanced topics

Loss functions: See references/loss-functions.md for sigmoid vs hinge loss, mathematical formulations, and when to use each.

Hyperparameter tuning: See references/hyperparameters.md for beta, gamma, learning rate selection guide, and model-size-specific recommendations.

Dataset preparation: See references/datasets.md for preference data formats, quality filtering, and custom dataset creation.

Hardware requirements

GPU: NVIDIA A100/H100 recommended
VRAM:
- 7B model: 1× A100 40GB (DeepSpeed ZeRO-3)
- 8B model: 2× A100 40GB
- 70B model: 8× A100 80GB
Single-node: DeepSpeed ZeRO-3 sufficient
Mixed precision: BF16 recommended

Memory optimization:

DeepSpeed ZeRO-3 (default config)
Gradient checkpointing
Flash Attention 2

Resources

Paper: https://arxiv.org/abs/2405.14734 (NeurIPS 2024)
GitHub: https://github.com/princeton-nlp/SimPO
Models: https://huggingface.co/princeton-nlp
Alignment Handbook: https://github.com/huggingface/alignment-handbook

Local tool required

1 FOR YOU

01
Set up the alignment-handbook environment
Create a Python 3.10 conda environment, install a matching PyTorch 2.2.2 build, clone and pip install huggingface/alignment-handbook, then install flash-attn before running the SimPO training scripts.

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SimPO - Simple Preference Optimization

Quick start

Common workflows

Workflow 1: Train from base model (Mistral 7B)

Workflow 2: Fine-tune instruct model (Llama 3 8B)

Workflow 3: Reasoning-intensive tasks (lower LR)

When to use vs alternatives

Common issues

Advanced topics

Hardware requirements

Resources

Local tool required

What's inside

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