Learning Dynamics of LLM Finetuning

Yi Ren, Danica J. Sutherland

Keywords: Learning dynamics, LLM, finetuning, DPO

TL;DR: The paper propose a novel learning dynamics framework to understand LLM's behavior during finetuning (e.g., SFT, DPO, and other variants). Some counter-intuitive behavior can be well explained by the proposed framework.

Abstract:

Learning dynamics, which describes how the learning of specific training examples influences the model's predictions on other examples,
gives us a powerful tool for understanding the behavior of deep learning systems. We study the learning dynamics of large language models during different types of finetuning, by analyzing the step-wise decomposition of how influence accumulates among different potential responses. Our framework allows a uniform interpretation of many interesting observations about the training of popular algorithms for both instruction tuning and preference tuning. In particular, we propose a hypothetical explanation of why specific types of hallucination are strengthened after finetuning, e.g., the model might use phrases or facts in the response for question B to answer question A, or the model might keep repeating similar simple phrases when generating responses. We also extend our framework and highlight a unique ``squeezing effect'' to explain a previously observed phenomenon in off-policy direct preference optimization (DPO), where running DPO for too long makes even the desired outputs less likely. This framework also provides insights into where the benefits of on-policy DPO and other variants come from. The analysis not only provides a novel perspective of understanding LLM's finetuning but also inspires a simple, effective method to improve alignment performance.

Primary Area: foundation or frontier models, including LLMs

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Submission Number: 4818