Proximal Policy Optimization (PPO) for LLMs Explained Intuitively

Video Description

In this video, I break down Proximal Policy Optimization (PPO) from first principles, without assuming prior knowledge of Reinforcement Learning. By the end, you’ll understand the core RL building blocks that led to PPO, including: 🔵 Policy Gradient 🔵 Actor-Critic Models 🔵 The Value Function 🔵 The Generalized Advantage Estimate In the LLM world, PPO was used to train reasoning models like OpenAI's o1/o3, and presumably Claude 3.7, Grok 3, etc. It’s the backbone of Reinforcement Learning with Human Feedback (RLHF) -- which helps align AI models with human preferences and Reinforcement Learning with Verifiable Rewards (RLVR), which gives LLMs reasoning abilities. Papers: - PPO paper: https://arxiv.org/pdf/1707.06347 - GAE paper: https://arxiv.org/pdf/1506.02438 - TRPO paper: https://arxiv.org/pdf/1502.05477 Well-written blogposts: - https://danieltakeshi.github.io/2017/04/02/notes-on-the-generalized-advantage-estimation-paper/ - https://huggingface.co/blog/NormalUhr/rlhf-pipeline - https://iclr-blog-track.github.io/2022/03/25/ppo-implementation-details/ Implementations: - (Original) OpenAI Baseslines: https://github.com/openai/baselines/blob/ea25b9e8b234e6ee1bca43083f8f3cf974143998/baselines/ppo2 - Hugging Face: https://github.com/huggingface/trl/blob/main/trl/trainer/ppo_trainer.py - Hugging Face docs: https://huggingface.co/docs/trl/main/en/ppo_trainer Mother of all RL books (Barto & Sutton): http://incompleteideas.net/book/RLbook2020.pdf 00:00 Intro 01:21 RL for LLMs 05:53 Policy Gradient 09:23 The Value Function 12:14 Generalized Advantage Estimate 17:17 End-to-end Training Algorithm 18:23 Importance Sampling 20:02 PPO Clipping 21:36 Outro Special thanks to Anish Tondwalkar for discussing some of these concepts with me. Note: At 21:10, A_t should have been inside the min. Thanks @t.w.7065 for catching this.