Group Relative Policy Optimization (GRPO)

Group Relative Policy Optimization (GRPO)

Overview

GRPO is a reinforcement learning method for LLM alignment. It generates multiple completions per prompt, scores them with a reward function, and optimizes the policy to favor higher-reward responses using relative policy gradients. This skill includes patterns for training thinking/​reasoning models.

Quick Reference

Critical Environment Setup

import os
from dotenv import load_dotenv
load_dotenv()

# Force text-based progress in Jupyter
os.environ["TQDM_NOTEBOOK"] = "false"

# CRITICAL: Set BEFORE importing unsloth/​TRL
os.environ['ACCELERATE_MIXED_PRECISION'] = 'bf16'

Critical Import Order

# CRITICAL: Import unsloth FIRST for proper TRL patching
import unsloth
from unsloth import FastLanguageModel, is_bf16_supported

# Then TRL imports
from trl import GRPOConfig, GRPOTrainer
from datasets import Dataset
import torch

Warning: Setting ACCELERATE_MIXED_PRECISION after imports may cause training issues.

GRPO Concepts

How GRPO Works

1. Generate multiple completions for each prompt
2. Score completions with reward function(s)
3. Compute relative advantages within each group
4. Update policy to favor higher-reward completions
5. Apply KL penalty to prevent divergence from reference

Key Differences from PPO

Setup

Load Model

from unsloth import FastLanguageModel

# Standard model
model, tokenizer = FastLanguageModel.from_pretrained(
    "unsloth/​Qwen3-4B-unsloth-bnb-4bit",
    max_seq_length=512,
    load_in_4bit=True,
)

# Thinking model (for reasoning tasks)
model, tokenizer = FastLanguageModel.from_pretrained(
    "unsloth/​Qwen3-4B-Thinking-2507-unsloth-bnb-4bit",
    max_seq_length=1024,  # Increased for thinking content
    load_in_4bit=True,
)

# Setup pad token (required for GRPO)
if tokenizer.pad_token is None:
    tokenizer.pad_token = tokenizer.eos_token
    tokenizer.pad_token_id = tokenizer.eos_token_id

Apply LoRA

model = FastLanguageModel.get_peft_model(
    model,
    r=16,
    lora_alpha=16,
    lora_dropout=0,
    target_modules=["q_proj", "k_proj", "v_proj", "o_proj",
                    "gate_proj", "up_proj", "down_proj"],
    use_gradient_checkpointing="unsloth",
)

Dataset Format

# GRPO requires prompts only (completions generated during training)
dataset = Dataset.from_dict({
    "prompt": [
        tokenizer.apply_chat_template(
            [{"role": "user", "content": "What is recursion?"}],
            tokenize=False, add_generation_prompt=True
        ),
        # ... more prompts
    ]
})

Reward Functions

Simple Reward Function

def length_reward(completions, prompts=None):
    """Reward based on response length."""
    rewards = []
    for completion in completions:
        length = len(completion.split())
        if length < 5:
            rewards.append(-1.0)
        elif length < 50:
            rewards.append(1.0)
        else:
            rewards.append(0.5)
    return rewards

LLM-as-Judge Reward

def llm_judge_reward(completions, prompts):
    """Use another LLM to score responses."""
    rewards = []
    for prompt, completion in zip(prompts, completions):
        score = judge_model.evaluate(prompt, completion)
        rewards.append(score)
    return rewards

Rule-Based Reward

def format_reward(completions, prompts=None):
    """Reward proper formatting."""
    rewards = []
    for completion in completions:
        score = 0.0
        if completion.endswith("."):
            score += 0.5
        if not completion.startswith(" "):
            score += 0.5
        rewards.append(score)
    return rewards

Composite Rewards

def combined_reward(completions, prompts):
    """Combine multiple reward signals."""
    length_scores = length_reward(completions)
    format_scores = format_reward(completions)
    return [0.5 * l + 0.5 * f for l, f in zip(length_scores, format_scores)]

Thinking-Aware Reward Function (Token-Based)

Use completion_ids parameter from TRL for efficient token-based parsing:

THINK_END_TOKEN_ID = 151668  # </​think> token for Qwen3-Thinking models

def thinking_reward_fn(completions, prompts=None, completion_ids=None, **kwargs):
    """
    Token-based reward function using completion_ids provided by TRL.

    Benefits over string matching:
    - No re-tokenization overhead (faster training)
    - Exact token boundaries (no regex edge cases)
    - Consistent with inference code pattern

    Scoring:
    - No </​think> token: -1.0 (strongly penalized)
    - Short thinking (<10 tokens): 0.3
    - Medium thinking (10-30 tokens): 0.7
    - Long thinking (>30 tokens): 1.0
    - Bonus +0.1 for self-questioning (contains '?')
    """
    rewards = []

    for completion, comp_ids in zip(completions, completion_ids):
        # Token-based detection using </​think> token ID
        if THINK_END_TOKEN_ID in comp_ids:
            end_idx = comp_ids.index(THINK_END_TOKEN_ID)
            thinking_length = end_idx  # Token count before </​think>

            # String-based content analysis for question detection
            thinking_content = completion.split('</​think>')[0]
            has_self_questions = '?' in thinking_content

            # Score based on thinking token count
            if thinking_length < 10:
                reward = 0.3  # Minimal thinking
            elif thinking_length < 30:
                reward = 0.7 + (0.1 if has_self_questions else 0)
            else:
                reward = 1.0 + (0.1 if has_self_questions else 0)
        else:
            reward = -1.0  # No </​think> token found

        rewards.append(reward)

    return rewards

Key insight: TRL passes completion_ids directly to reward functions, eliminating re-tokenization overhead.

Multi-Objective Thinking Reward (Token-Based)

THINK_END_TOKEN_ID = 151668  # </​think> token for Qwen3-Thinking models

def comprehensive_thinking_reward(completions, prompts=None, completion_ids=None, **kwargs):
    """
    Evaluate multiple aspects of thinking quality using token IDs.

    Scoring breakdown:
    - Has </​think> token: +0.3
    - Thinking depth (20+ tokens): +0.3
    - Structured sentences: +0.2
    - Self-questioning: +0.1
    - Step-by-step reasoning: +0.1
    """
    rewards = []

    for completion, comp_ids in zip(completions, completion_ids):
        score = 0.0

        # Token-based boundary detection
        if THINK_END_TOKEN_ID in comp_ids:
            score += 0.3  # Has proper </​think> token
            end_idx = comp_ids.index(THINK_END_TOKEN_ID)
            thinking_length = end_idx  # Token count

            # Extract thinking content for text analysis
            thinking = completion.split('</​think>')[0]

            # Depth (token count from IDs)
            if thinking_length >= 20:
                score += 0.3
            elif thinking_length >= 10:
                score += 0.2

            # Structure (sentences in text)
            sentences = thinking.count('.') + thinking.count('!')
            if sentences >= 2:
                score += 0.2

            # Self-questioning
            if '?' in thinking:
                score += 0.1

            # Step-by-step reasoning
            if any(w in thinking.lower() for w in ['first', 'then', 'next', 'finally']):
                score += 0.1
        else:
            score = -0.5  # Penalize missing </​think> token

        rewards.append(score)

    return rewards

GRPOTrainer Configuration

Basic Configuration

from trl import GRPOConfig

grpo_config = GRPOConfig(
    output_dir="./​grpo_output",
    per_device_train_batch_size=1,
    gradient_accumulation_steps=4,
    max_steps=100,
    learning_rate=1e-5,
    fp16=not is_bf16_supported(),
    bf16=is_bf16_supported(),
    optim="adamw_8bit",
    max_completion_length=128,
    num_generations=4,
    beta=0.1,
)

Key Parameters

Training

Basic Training Loop

from trl import GRPOTrainer

trainer = GRPOTrainer(
    model=model,
    args=grpo_config,
    train_dataset=dataset,
    processing_class=tokenizer,
    reward_funcs=length_reward,
)

trainer.train()

Multiple Reward Functions

trainer = GRPOTrainer(
    model=model,
    args=grpo_config,
    train_dataset=dataset,
    processing_class=tokenizer,
    reward_funcs=[length_reward, format_reward],
    reward_weights=[0.5, 0.5],
)

Troubleshooting

Reward Hacking

Symptom: Model exploits reward function (e.g., always outputs same length)

Fix:

• Add diversity penalties
• Use multiple reward signals
• Cap maximum reward

KL Divergence Too High

Symptom: Policy diverges too far from reference

Fix:

• Increase beta (stronger KL penalty)
• Reduce learning_rate
• Fewer training steps

Training Instability

Symptom: Loss spikes or NaN

Fix:

• Lower learning_rate to 5e-6
• Reduce num_generations to 2
• Check reward scale (should be roughly -1 to 1)

Memory Issues

Symptom: OOM with multiple generations

Fix:

• Reduce num_generations to 2
• Use gradient checkpointing
• Reduce max_completion_length

Kernel Shutdown (Jupyter)

GRPO training uses significant GPU memory. Shutdown kernel to release memory:

import IPython
print("Shutting down kernel to release GPU memory...")
app = IPython.Application.instance()
app.kernel.do_shutdown(restart=False)

Important: Always run this at the end of training notebooks before switching to different models.

When to Use This Skill

Use when:

• Aligning models with human preferences
• Optimizing for specific behaviors
• Post-SFT refinement
• Building reward-driven systems
• Simpler alternative to PPO

Cross-References

• bazzite-ai-jupyter:sft - Pre-training before GRPO
• bazzite-ai-jupyter:dpo - Simpler preference learning (no reward model)
• bazzite-ai-jupyter:rloo - Alternative RL method with lower variance
• bazzite-ai-jupyter:reward - Training reward models for GRPO
• bazzite-ai-jupyter:peft - LoRA for efficient RL
• bazzite-ai-jupyter:inference - Fast inference with vLLM
• bazzite-ai-ollama:api - Reward model inference