Post-trains LLMs with Hugging Face TRL trainers: SFT, DPO, reward modeling, PPO, GRPO, and RLHF pipelines on CUDA. Use when instruction-tuning, preference-aligning, or running online RL with a reward function. Not for HuggingFace Trainer-only SFT without preferences, Unsloth/Axolotl YAML stacks, or CPU-only training; never skip LoRA/QLoRA on 7B-class models without checking VRAM.
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Post-trains LLMs with Hugging Face TRL trainers: SFT, DPO, reward modeling, PPO, GRPO, and RLHF pipelines on CUDA. Use when instruction-tuning, preference-aligning, or running online RL with a reward function. Not for HuggingFace Trainer-only SFT without preferences, Unsloth/Axolotl YAML stacks, or CPU-only training; never skip LoRA/QLoRA on 7B-class models without checking VRAM.
TRL (Transformer Reinforcement Learning) is the HuggingFace library for post-training language models. It provides trainers for supervised fine-tuning (SFT), preference alignment (DPO and variants), reward modeling, and online reinforcement learning (PPO, GRPO, RLOO, OnlineDPO). This skill covers the full lifecycle from base model to human-aligned model, with copy-pasteable commands and progressive disclosure into reference files for advanced topics.
When to Use
Use this skill when any of the following apply:
You need to instruction-tune a base model with prompt-completion pairs → SFT
You have preference data (chosen/rejected pairs) and want alignment without a reward model → DPO
You are building a full RLHF pipeline (SFT → Reward Model → PPO)
You need to train a reward model to score generations
You want online RL with a custom reward function, especially under memory constraints → GRPO
You need PPO, RLOO, or OnlineDPO for maximum control over reinforcement learning
For GPU support ensure torch is installed with the correct CUDA build for your platform.
Hardware Requirements
Method
Model Size
Approx. VRAM
Notes
SFT (LoRA)
7B
16 GB
LoRA/QLoRA required
DPO
7B
24 GB
Stores reference model in memory
PPO
7B
40 GB
Policy + reward model + value model
GRPO
7B
24 GB
More memory-efficient than PPO
GPU: NVIDIA with CUDA required for all methods.
Multi-GPU: Supported via accelerate. Use accelerate launch for distributed training.
Mixed precision: BF16 recommended on A100/H100. Use FP16 on older architectures (V100, T4).
Memory optimization: Use LoRA/QLoRA for all methods, enable gradient checkpointing, reduce batch size with gradient accumulation to maintain effective batch.
Windows Host Notes
Primary development environment is Windows with PowerShell. When running CLI commands on Windows:
Use PowerShell syntax (backticks for line continuation, or keep commands on one line).
Paths use backslashes in PowerShell: ~\models\Qwen2.5-0.5B-SFT.
Load references/sft-training.md when you need: dataset format details, chat template configuration, packing strategies, multi-GPU training setup, or LoRA/QLoRA configuration for SFT.
Workflow 2: DPO — Preference Alignment Without a Reward Model
Align a model with chosen/rejected preference pairs. No reward model required.
Step 1 — Prepare preference dataset:
Required format (JSON):
{"prompt":"What is the capital of France?","chosen":"The capital of France is Paris.","rejected":"I don't know."}
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 1 — SFT: Follow Workflow 1 above. Save to Qwen2.5-0.5B-SFT.
Step 2 — Train reward model:
from transformers import AutoModelForSequenceClassification
from trl import RewardTrainer, RewardConfig
from datasets import load_dataset
model = AutoModelForSequenceClassification.from_pretrained(
"Qwen2.5-0.5B-SFT",
num_labels=1# Single reward score
)
tokenizer = AutoTokenizer.from_pretrained("Qwen2.5-0.5B-SFT")
dataset = load_dataset("trl-lib/ultrafeedback_binarized", split="train")
training_args = RewardConfig(
output_dir="Qwen2.5-0.5B-Reward",
per_device_train_batch_size=2,
num_train_epochs=1,
learning_rate=1e-5
)
trainer = RewardTrainer(
model=model,
args=training_args,
processing_class=tokenizer,
train_dataset=dataset
)
trainer.train()
trainer.save_model()
Load references/reward-modeling.md when you need: outcome vs process reward models, Bradley-Terry loss details, reward model evaluation metrics, or reward hacking mitigation.
Load references/online-rl.md when you need: PPO, RLOO, or OnlineDPO detailed configurations, KL coefficient tuning, or multi-GPU online RL setup.
Step 4 — Evaluate:
from transformers import pipeline
generator = pipeline("text-generation", model="Qwen2.5-0.5B-PPO")
prompt = "Explain quantum computing to a 10-year-old"
output = generator(prompt, max_length=200)[0]["generated_text"]
print(output)
Workflow 4: GRPO — Memory-Efficient Online RL
Train with reinforcement learning using a custom reward function. GRPO is more memory-efficient than PPO because it does not require a separate value model.
Load references/grpo-training.md when you need: reward function design philosophy, training insights (why loss increases, mode collapse detection), hyperparameter tuning, multi-stage training, or troubleshooting. A production-ready training script is in templates/basic_grpo_training.py.
Step 1 — Define reward function:
defreward_function(completions, **kwargs):
"""
Compute rewards for completions.
Args:
completions: List of generated texts
Returns:
List of reward scores (floats)
"""
rewards = []
for completion in completions:
score = len(completion.split()) # Favor longer responses
score += len(set(completion.lower().split())) # Reward unique words
rewards.append(score)
return rewards
Or use a trained reward model:
from transformers import pipeline
reward_model = pipeline("text-classification", model="reward-model-path")
defreward_from_model(completions, prompts, **kwargs):
full_texts = [p + c for p, c inzip(prompts, completions)]
results = reward_model(full_texts)
return [r["score"] for r in results]
Step 2 — Configure GRPO:
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
)
DPO stores both the policy and a frozen reference model in memory. Reduce batch size and sequence length, or enable gradient checkpointing:
config = DPOConfig(
per_device_train_batch_size=1, # Reduce from 4
max_length=512, # Reduce from 1024
gradient_accumulation_steps=8# Maintain effective batch
)
model.gradient_checkpointing_enable()
Poor DPO alignment quality
The beta parameter controls the KL penalty strength. Tune it:
Higher beta (e.g. 0.5) = more conservative, stays closer to reference model.
Lower beta (e.g. 0.01) = more aggressive alignment, risk of over-optimization.
config = DPOConfig(beta=0.5) # More conservative
config = DPOConfig(beta=0.01) # More aggressive
Reward model not learning
Check loss type and learning rate. Reward models often need lower LR and more epochs:
config = RewardConfig(
learning_rate=1e-5, # Try different LR
num_train_epochs=3# Train longer
)
Verify your preference dataset has clear winners:
print(dataset[0])
# Should show clear chosen > rejected quality difference
PPO training unstable
Adjust KL coefficient and clip range:
config = PPOConfig(
kl_coef=0.1, # Increase from 0.05 to stabilize
cliprange=0.1# Reduce from 0.2 to limit policy updates
)
GRPO loss increasing
GRPO loss can increase during training — this is expected behavior and does not necessarily indicate a problem. The loss in RL is not directly comparable to supervised losses. See references/grpo-training.md for detailed explanation and mode collapse detection.
General memory optimization
Use LoRA/QLoRA for all methods to drastically reduce VRAM.
Reduce per_device_train_batch_size and compensate with gradient_accumulation_steps.
Use BF16 mixed precision on A100/H100.
Verification
Verify SFT model was saved
from transformers import AutoModelForCausalLM, AutoTokenizer
model = AutoModelForCausalLM.from_pretrained("Qwen2.5-0.5B-SFT")
tokenizer = AutoTokenizer.from_pretrained("Qwen2.5-0.5B-SFT")
print("Model loaded successfully:", model.config.model_type)
Verify DPO model generates aligned outputs
from transformers import pipeline
generator = pipeline("text-generation", model="Qwen2.5-0.5B-DPO", tokenizer="Qwen2.5-0.5B-DPO")
output = generator("What is the capital of France?", max_new_tokens=50)[0]["generated_text"]
print(output)
# Expect: response aligned with preference data style
Verify reward model produces scores
from transformers import pipeline
reward_pipe = pipeline("text-classification", model="Qwen2.5-0.5B-Reward")
score = reward_pipe("The capital of France is Paris.")
print("Reward score:", score)
# Expect: a float score in the "score" field
Verify training logs
Check that loss is decreasing (SFT, DPO, Reward Model) or that reward is increasing (PPO, GRPO):
# Check trainer_state.json in output directorycat Qwen2.5-0.5B-SFT/trainer_state.json | python -m json.tool | head -20
import json
withopen("Qwen2-GRPO/trainer_state.json") as f:
state = json.load(f)
rewards = [entry.get("reward") for entry in state["log_history"] if"reward"in entry]
print("Reward trend:", rewards)
# Expect: generally increasing reward over training steps
Related Skills
peft-lora — LoRA/QLoRA configuration for memory-efficient fine-tuning
huggingface-training — Base HuggingFace Trainer for non-RL fine-tuning
axolotl-training — YAML-based training configuration alternative
unsloth-finetuning — Fast LoRA training for single-GPU scenarios
