TRL - Transformer Reinforcement Learning

Quick start

TRL provides post-training methods for aligning language models with human preferences.

Installation:

bash
pip install trl transformers datasets peft accelerate

Supervised Fine-Tuning (instruction tuning):

python
from trl import SFTTrainer

trainer = SFTTrainer(
    model="Qwen/Qwen2.5-0.5B",
    train_dataset=dataset,  # Prompt-completion pairs
)
trainer.train()

DPO (align with preferences):

python
from trl import DPOTrainer, DPOConfig

config = DPOConfig(output_dir="model-dpo", beta=0.1)
trainer = DPOTrainer(
    model=model,
    args=config,
    train_dataset=preference_dataset,  # chosen/rejected pairs
    processing_class=tokenizer
)
trainer.train()

Common workflows

Workflow 1: Full RLHF pipeline (SFT → Reward Model → PPO)

Complete pipeline from base model to human-aligned model.

Copy this checklist:

RLHF Training:
- [ ] Step 1: Supervised fine-tuning (SFT)
- [ ] Step 2: Train reward model
- [ ] Step 3: PPO reinforcement learning
- [ ] Step 4: Evaluate aligned model

Step 1: Supervised fine-tuning

Train base model on instruction-following data:

python
from transformers import AutoModelForCausalLM, AutoTokenizer
from trl import SFTTrainer, SFTConfig
from datasets import load_dataset

# Load model
model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen2.5-0.5B")
tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-0.5B")

# Load instruction dataset
dataset = load_dataset("trl-lib/Capybara", split="train")

# Configure training
training_args = SFTConfig(
    output_dir="Qwen2.5-0.5B-SFT",
    per_device_train_batch_size=4,
    num_train_epochs=1,
    learning_rate=2e-5,
    logging_steps=10,
    save_strategy="epoch"
)

# Train
trainer = SFTTrainer(
    model=model,
    args=training_args,
    train_dataset=dataset,
    tokenizer=tokenizer
)
trainer.train()
trainer.save_model()

Step 2: Train reward model

Train model to predict human preferences:

python
from transformers import AutoModelForSequenceClassification
from trl import RewardTrainer, RewardConfig

# Load SFT model as base
model = AutoModelForSequenceClassification.from_pretrained(
    "Qwen2.5-0.5B-SFT",
    num_labels=1  # Single reward score
)
tokenizer = AutoTokenizer.from_pretrained("Qwen2.5-0.5B-SFT")

# Load preference data (chosen/rejected pairs)
dataset = load_dataset("trl-lib/ultrafeedback_binarized", split="train")

# Configure training
training_args = RewardConfig(
    output_dir="Qwen2.5-0.5B-Reward",
    per_device_train_batch_size=2,
    num_train_epochs=1,
    learning_rate=1e-5
)

# Train reward model
trainer = RewardTrainer(
    model=model,
    args=training_args,
    processing_class=tokenizer,
    train_dataset=dataset
)
trainer.train()
trainer.save_model()

Step 3: PPO reinforcement learning

Optimize policy using reward model:

bash
python -m trl.scripts.ppo \
    --model_name_or_path Qwen2.5-0.5B-SFT \
    --reward_model_path Qwen2.5-0.5B-Reward \
    --dataset_name trl-internal-testing/descriptiveness-sentiment-trl-style \
    --output_dir Qwen2.5-0.5B-PPO \
    --learning_rate 3e-6 \
    --per_device_train_batch_size 64 \
    --total_episodes 10000

Step 4: Evaluate

python
from transformers import pipeline

# Load aligned model
generator = pipeline("text-generation", model="Qwen2.5-0.5B-PPO")

# Test
prompt = "Explain quantum computing to a 10-year-old"
output = generator(prompt, max_length=200)[0]["generated_text"]
print(output)

Workflow 2: Simple preference alignment with DPO

Align model with preferences without reward model.

Copy this checklist:

DPO Training:
- [ ] Step 1: Prepare preference dataset
- [ ] Step 2: Configure DPO
- [ ] Step 3: Train with DPOTrainer
- [ ] Step 4: Evaluate alignment

Step 1: Prepare preference dataset

Dataset format:

json
{
  "prompt": "What is the capital of France?",
  "chosen": "The capital of France is Paris.",
  "rejected": "I don't know."
}

Load dataset:

python
from datasets import load_dataset

dataset = load_dataset("trl-lib/ultrafeedback_binarized", split="train")
# Or load your own
# dataset = load_dataset("json", data_files="preferences.json")

Step 2: Configure DPO

python
from trl import DPOConfig

config = DPOConfig(
    output_dir="Qwen2.5-0.5B-DPO",
    per_device_train_batch_size=4,
    num_train_epochs=1,
    learning_rate=5e-7,
    beta=0.1,  # KL penalty strength
    max_prompt_length=512,
    max_length=1024,
    logging_steps=10
)

Step 3: Train with DPOTrainer

python
from transformers import AutoModelForCausalLM, AutoTokenizer
from trl import DPOTrainer

model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen2.5-0.5B-Instruct")
tokenizer = AutoTokenizer.from_pretrained("Qwen/Qwen2.5-0.5B-Instruct")

trainer = DPOTrainer(
    model=model,
    args=config,
    train_dataset=dataset,
    processing_class=tokenizer
)

trainer.train()
trainer.save_model()

CLI alternative:

bash
trl dpo \
    --model_name_or_path Qwen/Qwen2.5-0.5B-Instruct \
    --dataset_name argilla/Capybara-Preferences \
    --output_dir Qwen2.5-0.5B-DPO \
    --per_device_train_batch_size 4 \
    --learning_rate 5e-7 \
    --beta 0.1

Workflow 3: Memory-efficient online RL with GRPO

Train with reinforcement learning using minimal memory.

Copy this checklist:

GRPO Training:
- [ ] Step 1: Define reward function
- [ ] Step 2: Configure GRPO
- [ ] Step 3: Train with GRPOTrainer

Step 1: Define reward function

python
def reward_function(completions, **kwargs):
    """
    Compute rewards for completions.

    Args:
        completions: List of generated texts

    Returns:
        List of reward scores (floats)
    """
    rewards = []
    for completion in completions:
        # Example: reward based on length and unique words
        score = len(completion.split())  # Favor longer responses
        score += len(set(completion.lower().split()))  # Reward unique words
        rewards.append(score)
    return rewards

Or use a reward model:

python
from transformers import pipeline

reward_model = pipeline("text-classification", model="reward-model-path")

def reward_from_model(completions, prompts, **kwargs):
    # Combine prompt + completion
    full_texts = [p + c for p, c in zip(prompts, completions)]
    # Get reward scores
    results = reward_model(full_texts)
    return [r["score"] for r in results]

Step 2: Configure GRPO

python
from trl import GRPOConfig

config = GRPOConfig(
    output_dir="Qwen2-GRPO",
    per_device_train_batch_size=4,
    num_train_epochs=1,
    learning_rate=1e-5,
    num_generations=4,  # Generate 4 completions per prompt
    max_new_tokens=128
)

Step 3: Train with GRPOTrainer

python
from datasets import load_dataset
from trl import GRPOTrainer

# Load prompt-only dataset
dataset = load_dataset("trl-lib/tldr", split="train")

trainer = GRPOTrainer(
    model="Qwen/Qwen2-0.5B-Instruct",
    reward_funcs=reward_function,  # Your reward function
    args=config,
    train_dataset=dataset
)

trainer.train()

CLI:

bash
trl grpo \
    --model_name_or_path Qwen/Qwen2-0.5B-Instruct \
    --dataset_name trl-lib/tldr \
    --output_dir Qwen2-GRPO \
    --num_generations 4

When to use vs alternatives

Use TRL when:

• Need to align model with human preferences
• Have preference data (chosen/rejected pairs)
• Want to use reinforcement learning (PPO, GRPO)
• Need reward model training
• Doing RLHF (full pipeline)

Method selection:

• SFT: Have prompt-completion pairs, want basic instruction following
• DPO: Have preferences, want simple alignment (no reward model needed)
• PPO: Have reward model, need maximum control over RL
• GRPO: Memory-constrained, want online RL
• Reward Model: Building RLHF pipeline, need to score generations

Use alternatives instead:

• HuggingFace Trainer: Basic fine-tuning without RL
• Axolotl: YAML-based training configuration
• LitGPT: Educational, minimal fine-tuning
• Unsloth: Fast LoRA training

Common issues

Issue: OOM during DPO training

Reduce batch size and sequence length:

python
config = DPOConfig(
    per_device_train_batch_size=1,  # Reduce from 4
    max_length=512,  # Reduce from 1024
    gradient_accumulation_steps=8  # Maintain effective batch
)

Or use gradient checkpointing:

python
model.gradient_checkpointing_enable()

Issue: Poor alignment quality

Tune beta parameter:

python
# Higher beta = more conservative (stays closer to reference)
config = DPOConfig(beta=0.5)  # Default 0.1

# Lower beta = more aggressive alignment
config = DPOConfig(beta=0.01)

Issue: Reward model not learning

Check loss type and learning rate:

python
config = RewardConfig(
    learning_rate=1e-5,  # Try different LR
    num_train_epochs=3  # Train longer
)

Ensure preference dataset has clear winners:

python
# Verify dataset
print(dataset[0])
# Should have clear chosen > rejected

Issue: PPO training unstable

Adjust KL coefficient:

python
config = PPOConfig(
    kl_coef=0.1,  # Increase from 0.05
    cliprange=0.1  # Reduce from 0.2
)

Advanced topics

SFT training guide: See references/sft-training.md for dataset formats, chat templates, packing strategies, and multi-GPU training.

DPO variants: See references/dpo-variants.md for IPO, cDPO, RPO, and other DPO loss functions with recommended hyperparameters.

Reward modeling: See references/reward-modeling.md for outcome vs process rewards, Bradley-Terry loss, and reward model evaluation.

Online RL methods: See references/online-rl.md for PPO, GRPO, RLOO, and OnlineDPO with detailed configurations.

Hardware requirements

• GPU: NVIDIA (CUDA required)
• VRAM: Depends on model and method
• SFT 7B: 16GB (with LoRA)
• DPO 7B: 24GB (stores reference model)
• PPO 7B: 40GB (policy + reward model)
• GRPO 7B: 24GB (more memory efficient)
• Multi-GPU: Supported via accelerate
• Mixed precision: BF16 recommended (A100/H100)

Memory optimization:

• Use LoRA/QLoRA for all methods
• Enable gradient checkpointing
• Use smaller batch sizes with gradient accumulation

Resources

• Docs: https://huggingface.co/docs/trl/
• GitHub: https://github.com/huggingface/trl
• Papers:
• "Training language models to follow instructions with human feedback" (InstructGPT, 2022)
• "Direct Preference Optimization: Your Language Model is Secretly a Reward Model" (DPO, 2023)
• "Group Relative Policy Optimization" (GRPO, 2024)
• Examples: https://github.com/huggingface/trl/tree/main/examples/scripts