SFT, RL and DPO: The Other Stack

Post-training is the part that produces the weights everything else in this series then has to serve. Different tooling, different failure modes, and one connection to the serving stack that decides how long a training run takes.

A bonus chapter, not part eight. Everything below the arrow is the rest of this series.

• Part 1: Start Here: The Words Everyone Uses About LLM Inference
• Part 2: Your KV Cache Is Bigger Than Your Model
• Part 3: What a Kernel Is, and Why Everyone Is Writing New Ones
• Part 4: Quantization Is Four Decisions, Not One
• Part 5: Tuning vLLM: What Every Setting Does to the Arithmetic
• Part 6: Your Second GPU Is Bought for the Cache, Not the Model
• Part 7: Your Context Length Decides What a Kernel Is Worth

This series has been about serving a model. What it costs, where the time goes, which settings matter. All of it assumed the model already existed. Somebody handed you a set of weights and your job started there.

Post-training is where those weights got their behaviour. SFT, DPO, PPO, GRPO and RLVR…