提升零阶优化稳定性与收敛速度,适用于无梯度场景。
Refining Adaptive Zeroth-Order Optimization at Ease
- 利用一阶矩方差缩减改进零阶梯度估计精度
- 重构二阶矩估计,更准确捕捉优化几何结构
- 理论与实验均证明优于现有方法,适合资源受限场景
零阶(ZO)优化在无法获取梯度信息的黑盒系统和资源受限环境中至关重要。尽管现有自适应方法如ZO-AdaMM表现良好,但因未能充分利用一阶矩信息,导致收敛性能受限。本文提出改进的自适应零阶优化方法R-AdaZO:首先揭示一阶矩估计对零阶梯度估计具有未被挖掘的方差缩减效应,提升了更新的准确性和稳定性;进而基于此方差缩减后的梯度,重构二阶矩估计,更精确地刻画优化曲面几何,实现更有效的更新缩放。理论分析首次揭示了零阶优化中一阶矩的方差缩减机制,给出了更接近无偏理想的二阶矩近似,并建立首个针对自适应零阶方法的方差感知收敛框架。实验验证了R-AdaZO在合成问题、黑盒对抗攻击及大语言模型低内存微调中的优越收敛性,表明其为现实零阶优化挑战提供了更优解。
原文摘要 · Abstract (English)
Recently, zeroth-order (ZO) optimization plays an essential role in scenarios where gradient information is inaccessible or unaffordable, such as black-box systems and resource-constrained environments. While existing adaptive methods such as ZO-AdaMM have shown promise, they are fundamentally limited by their underutilization of moment information during optimization, usually resulting in underperforming convergence. To overcome these limitations, this paper introduces Refined Adaptive Zeroth-Order Optimization (R-AdaZO). Specifically, we first show the untapped variance reduction effect of first moment estimate on ZO gradient estimation, which improves the accuracy and stability of ZO updates. We then refine the second moment estimate based on these variance-reduced gradient estimates to better capture the geometry of the optimization landscape, enabling a more effective scaling of ZO updates. We present rigorous theoretical analysis to show (a) the first analysis to the variance reduction of first moment estimate in ZO optimization, (b) the improved second moment estimates with a more accurate approximation of its variance-free ideal, (c) the first variance-aware convergence framework for adaptive ZO methods, which may be of independent interest, and (d) the faster convergence of R-AdaZO than existing baselines like ZO-AdaMM. Our extensive experiments, including synthetic problems, black-box adversarial attack, and memory-efficient fine-tuning of large language models (LLMs), further verify the superior convergence of R-AdaZO, indicating that R-AdaZO offers an improved solution for real-world ZO optimization challenges.
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