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arXiv · 2602.13069

Memory-Efficient Structured Backpropagation for On-Device LLM Fine-Tuning

Juneyoung Park·Yuri Hong·Seongwan Kim·Jaeho Lee
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Abstract

On-device fine-tuning enables privacy-preserving personalization of large language models, but mobile devices impose severe memory constraints, typically 6--12GB shared across all workloads. Existing approaches force a trade-off between exact gradients with high memory (MeBP) and low memory with noisy estimates (MeZO). We propose Memory-efficient Structured Backpropagation (MeSP), which bridges this gap by manually deriving backward passes that exploit LoRA's low-rank structure. Our key insight is that the intermediate projection $h = xA$ can be recomputed during backward at minimal cost since rank $r \ll d_{in}$, eliminating the need to store it. MeSP achieves 49\% average memory reduction compared to MeBP on Qwen2.5 models (0.5B--3B) while computing mathematically identical gradients. Our analysis also reveals that MeZO's gradient estimates show near-zero correlation with true gradients (cosine similarity $\approx$0.001), explaining its slow convergence. MeSP reduces peak memory from 361MB to 136MB for Qwen2.5-0.5B, enabling fine-tuning scenarios previously infeasible on memory-constrained devices.

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Juneyoung Park, Yuri Hong, Seongwan Kim, Jaeho Lee. 2026-05-29. Memory-Efficient Structured Backpropagation for On-Device LLM Fine-Tuning. https://arxiv.org/abs/2602.13069

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