PeriodicLoRA: Breaking the Low-Rank Bottleneck in LoRA Optimization

TL;DR

PeriodicLoRA enhances low-rank fine-tuning of large language models by accumulating updates over multiple stages, significantly improving performance without additional memory costs.

Contribution

It introduces a novel periodic accumulation method for LoRA, breaking the low-rank bottleneck and boosting fine-tuning effectiveness.

Findings

PLoRA achieves approximately 1.8 times the learning ability of standard LoRA.

It improves fine-tuning performance without increasing memory usage.

A momentum-based unloading strategy stabilizes training.

Abstract

Supervised fine-tuning is the most common method to adapt large language models (LLMs) to downstream tasks, but full fine-tuning LLMs requires massive computational resources. Recently, parameter-efficient fine-tuning (PEFT) methods have been widely studied due to its cost-effectiveness. LoRA is one of the most widely used methods, which assumes that the optimization process is essentially low-dimensional. Although LoRA fine-tuning is effective, there is still a performance gap compared to full fine-tuning, since its weight update is limited to low-rank matrices. In order to break the low-rank bottleneck in LoRA Optimization, we propose PeriodicLoRA (PLoRA), which accumulates low-rank update matrices multiple times to achieve a higher update rank. PLoRA has multiple training stages. During each stage, we still update only the LoRA weights. However, at the end of each stage, we unload the LoRA weights into the backbone parameters and then reinitialize the LoRA states. Experimental results show that PLoRA has stronger learning ability, approximately 1.8 times that of LoRA's learning ability at most, but it does not increase memory usage. Further, we introduce a momentum-based unloading strategy for PLoRA to mitigate the training instability.