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LLMs 37. Large Language Models (LLMs) — Reinforcement Learning (RL) Section

This conceptual guide covers reinforcement learning for large language models, specifically RLHF and its variants, the place of RL in the LLM training…

Large Language Models (LLMs) Reinforcement Learning - RLHF and Its Variants

A Learning-Oriented Guide to the Full Alignment Landscape

This post is designed as a conceptual learning resource rather than a paper summary or implementation guide. The goal is to help you understand why RLHF exists, how it fits into the broader LLM training pipeline, where it breaks down, and why the field is rapidly moving toward simpler and more data-centric alternatives. If you internalize the ideas here, most RLHF-related papers will feel like variations on a small number of core themes.

I. The Classic LLM Training Pipeline

Modern decoder-only LLMs such as ChatGPT, LLaMA, and Baichuan follow a broadly shared training paradigm. The model does not become a “chatbot” all at once. Instead, it is gradually transformed through stages that serve very different purposes.

The pipeline can be summarized as:

Pre-training: learn language and world knowledge
Supervised Fine-Tuning (SFT): learn how to follow instructions
Alignment: learn how to behave in ways humans prefer

Each stage builds on the previous one. Importantly, later stages rarely add new intelligence - they reshape how existing intelligence is expressed.

II. Pre-training: Where Capability Is Born

Pre-training is the most expensive and most important phase. The model is trained on hundreds of billions or trillions of tokens to predict the next token given context. This simple objective is powerful enough to induce grammar, reasoning patterns, factual knowledge, and abstractions.

At this stage, the model is not helpful or safe. It is best thought of as a general-purpose pattern learner. Most of what we later call “reasoning ability” is already present after pre-training.

This insight is crucial, because it explains why alignment methods can work with surprisingly small amounts of data.

III. Supervised Fine-Tuning (SFT): Learning to Follow Instructions

Supervised Fine-Tuning introduces human intent into the system.

The training format changes from raw text to instruction–response pairs. The objective is still next-token prediction, but the data distribution is different. The model learns that certain inputs correspond to certain styles of output: explanations, summaries, code, or dialogue.

The key differences from pre-training are:

the dataset is much smaller,
the data is human-curated,
the model is learning behavior, not new facts.

SFT teaches the model how to respond, but it does not guarantee that the responses are helpful, safe, or honest in edge cases.

IV. Alignment: Shaping Behavior, Not Knowledge

Alignment is about making the model’s outputs conform to human preferences and values. This includes helpfulness, honesty, harmlessness, and consistency over long interactions.

This is where reinforcement learning ideas enter the picture. Alignment is fundamentally about preference optimization under uncertainty, which is difficult to capture with supervised labels alone.

V. Reinforcement Learning from Human Feedback (RLHF)

The Core RLHF Workflow

RLHF operationalizes alignment through three steps:

Start from an SFT model
Train a reward model (RM) that predicts human preferences
Optimize the SFT model using PPO, guided by the reward model

The reward model acts as a proxy for human judgment, converting qualitative preferences into a scalar signal that can be optimized.

How the Reward Model Is Built

For each prompt, the SFT model generates multiple candidate responses. Humans rank these responses. The rankings are then converted into training data for the reward model.

The reward model is typically the SFT model plus a regression head that outputs a scalar reward. This scalar is treated as “how good” a response is according to human preference.

This step is expensive, but still cheaper than writing high-quality responses from scratch.

PPO: Optimizing with a Proxy Reward

PPO updates the policy model so that it produces higher-reward outputs while staying close to the original SFT distribution. The closeness is enforced using a KL-divergence penalty.

Conceptually:

the model explores new behaviors,
the reward model scores them,
PPO nudges the model toward higher-reward regions,
but prevents it from drifting too far and “gaming” the reward.

This balance is delicate and central to RLHF’s complexity.

VI. RLHF in LLaMA 2: Key Design Differences

LLaMA 2 introduces several refinements over the original InstructGPT-style RLHF.

Margin Loss for Preference Strength

Instead of treating all preferences equally, LLaMA 2 distinguishes between “slightly better” and “significantly better” responses. This is implemented via a margin loss, which increases gradient strength when preferences are strong.

This accelerates learning and improves stability.

Two Reward Models: Helpfulness and Safety

Rather than collapsing all preferences into one signal, LLaMA 2 trains:

a helpfulness reward model,
a safety reward model.

The final reward is a weighted combination. This separation makes trade-offs explicit and easier to control.

Rejection Sampling + PPO

LLaMA 2 combines PPO with rejection sampling (best-of-n). The model generates multiple candidates, selects the highest-reward one, and uses it for training. PPO is then applied on top of this process.

This hybrid approach improves sample efficiency and stabilizes training.

VII. Why Alternatives to RLHF Are Needed

RLHF works, but it is expensive, slow, and complex.

Human preference data is costly
PPO requires multiple large models simultaneously
Training cycles are long
Reward hacking and over-optimization are real risks

These drawbacks motivated a wave of alternative alignment methods.

VIII. Major Alternatives to RLHF

Constitutional AI

Instead of human rankings, Constitutional AI uses a set of explicit rules (a “constitution”) and lets AI models critique and revise their own outputs based on those rules. Reinforcement learning is still used, but humans define principles rather than labels.

Hindsight Instruction Relabeling (HIR)

HIR reframes failures as learning opportunities. When a model fails to follow an instruction, the instruction is relabeled to match what the model actually did well. This converts failed attempts into useful supervised data.

The key insight: not all mistakes require reinforcement learning.

Direct Preference Optimization (DPO)

DPO removes the reward model and PPO entirely. It shows that preference comparisons can be optimized directly using a likelihood-based objective.

This dramatically simplifies the pipeline and often outperforms PPO-based RLHF in practice.

Reinforced Self-Training (ReST)

ReST iteratively generates data, filters it using preference signals, and retrains the model on increasingly higher-quality subsets. It is more offline and data-driven than PPO-based RLHF.

RLAIF: AI Replacing Humans

RLAIF replaces human feedback with AI-generated feedback. Surprisingly, performance is often close to human-based RLHF, especially for safety and truthfulness tasks.

This suggests that current LLMs already encode strong alignment priors.

IX. RLHF Experiments: Choosing the Best Checkpoint

A critical insight from RLHF experiments is that reward model scores are misleading.

As training progresses:

proxy reward keeps increasing,
true performance increases, then decreases.

The divergence between proxy reward and true reward is controlled by KL divergence from the initial model. OpenAI observed that true reward often follows a predictable curve as a function of KL divergence.

This leads to practical heuristics for checkpoint selection, rather than blindly maximizing reward model output.

X. Scaling Laws and What They Teach Us

Empirical results show that:

larger reward models support larger safe policy updates,
reward model dataset size matters more than model size below a threshold,
larger policy models benefit less from RLHF because they start stronger.

Perhaps the most surprising finding is that reward collapse happens at similar KL divergence values across model sizes.

This suggests that alignment is constrained by structure, not scale.

Final Perspective

RLHF is not a final solution - it is a transitional technology.

The trajectory of the field is clear:

from reinforcement learning → preference optimization,
from complex pipelines → simpler objectives,
from human-heavy workflows → data- and model-centric alignment.

The deeper lesson is this: Alignment is about distribution shaping, not capability creation.

Once you understand that, RLHF and all its variants become much easier to reason about - and easier to replace.

How to Use This Page as a Prep Plan

Do not treat this as passive reading. Convert the ideas in this page into a short weekly loop: learn one idea, practice it under interview conditions, then write down what changed. That is the fastest way to turn advice into visible interview behavior.

Prep area	What you need to prove	Practice artifact
Understand	Turn the prompt into a concrete goal.	Clarifying questions and success criteria.
Practice	Use realistic constraints and timed reps.	Worked examples with edge cases.
Explain	Make reasoning visible.	Tradeoffs, assumptions, and test strategy.
Improve	Review misses quickly.	A short feedback log and next action.

For LLMs 37. Large Language Models (LLMs) - Reinforcement Learning (RL) Section, the strongest candidates usually do three things well: they make their assumptions explicit, they use concrete examples instead of vague claims, and they review mistakes quickly enough that the next practice rep is better than the last one.

Video Walkthrough

This verified YouTube video gives a second pass on the same preparation area. Use it after reading the guide, then come back and turn the advice into a practice artifact.

Video: Reinforcement Learning from Human Feedback: From Zero to chatGPT

FAQ

How should I use this guide?

Read it once for the structure, then turn each section into a practice task with a visible artifact.

What should I do if I am short on time?

Prioritize the skills most likely to be tested, then do one mock or timed drill to expose the largest gap.

How do I know I am ready?

You can explain your approach clearly, recover from hints, and name tradeoffs without relying on memorized wording.