Re-examining Low Rank adaptation for private LLM fine-tuning
重新审视用于私有LLM微调的低秩适应
Ali Dadsetan, Frank Rudzicz
AI总结 研究差分隐私SGD中噪声导致的梯度奇异值膨胀问题,提出通过部分恢复原始奇异值分布来提升DP-SGD的样本效率。
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隐私是在敏感数据上微调大型语言模型(LLM)时的核心关注点,差分隐私随机梯度下降(DP-SGD)——它裁剪每个样本的梯度并添加校准的高斯噪声——是形式化隐私保证的标准工具。理论和实践都表明,低秩模型更适合DP训练,这一特性对LLM尤其相关,因为其微调梯度表现出强烈的低秩结构。诸如DP-LoRA之类的方法通过将更新限制在低秩子空间来利用这一点,即仅保留每层梯度SVD中的少数非零分量。然而,我们认为,虽然非零分量少很重要,但DP-SGD注入的各向同性噪声会膨胀梯度矩阵的奇异值,破坏其自然快速衰减。在这项工作中,我们研究了这种噪声引起的特征值膨胀是否会降低性能,并表明部分恢复原始奇异值分布显著提高了DP-SGD的样本效率。在语言分类(使用RoBERTa的GLUE基准)和文本生成(使用Qwen和Llama模型(参数高达4B)的E2E和DART表格到文本基准)上的实验表明,恢复奇异值的快速衰减是一种在不损害隐私保证的情况下加速DP优化过程的有效策略。
Privacy is a central concern when fine-tuning large language models (LLMs) on sensitive data, and differentially private stochastic gradient descent (DP-SGD) -- which clips per-sample gradients and adds calibrated Gaussian noise -- is the standard tool for formal privacy guarantees. Both theory and practice show that lower-rank models are better suited to DP training, a property especially relevant for LLMs, whose fine-tuning gradients exhibit a strong low-rank structure. Methods such as DP-LoRA exploit this by restricting updates to a low-rank subspace, i.e., retaining only a few non-zero components in the SVD of each layer's gradient. However, we argue that while having few non-zero components is important, the isotropic noise injected by DP-SGD inflates the singular values of the gradient matrix, disrupting their naturally fast decay. In this work, we investigate whether this noise-induced eigenvalue blow-up reduces performance, and show that partially restoring the original singular-value profile significantly improves the sample efficiency of DP-SGD. Experiments on language classification (GLUE benchmark with RoBERTa) and text generation (E2E and DART table-to-text benchmarks with Qwen and Llama models up to 4B parameters) showcase that restoring the fast decay of singular values is a viable strategy for speeding up the DP optimization process, without compromising privacy guarantees.