Scaling Laws for Forgetting When Fine-Tuning Large Language Models
Damjan Kalajdzievski
We study and quantify the problem of forgetting when fine-tuning pre-trained
large language models (LLMs) on a downstream task. We find that
parameter-efficient fine-tuning (PEFT) strategies, such as Low-Rank Adapters
(LoRA), still suffer from catastrophic forgetting. In particular, we identify a
strong inverse linear relationship between the fine-tuning performance and the
amount of forgetting when fine-tuning LLMs with LoRA. We further obtain precise
scaling laws that show forgetting increases as a shifted power law in the
number of parameters fine-tuned and the number of update steps. We also examine
the impact of forgetting on knowledge, reasoning, and the safety guardrails
trained into Llama 2 7B chat. Our study suggests that forgetting cannot be
avoided through early stopping or by varying the number of parameters
fine-tuned. We believe this opens up an important safety-critical direction for
future research to evaluate and develop fine-tuning schemes which mitigate
forgetting