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原文摘要

Balancing convergence efficiency and robustness under Differential Privacy (DP) is a central challenge in Federated Learning (FL). Although AdamW accelerates training and fine-tuning in large-scale models, we find that directly applying it to Differentially Private FL (DPFL) suffers from three major issues: (i) data heterogeneity and privacy noise jointly amplify the variance of the second-moment estimator, (ii) DP perturbations bias the second-moment estimator, and (iii) DP amplifies AdamW's sensitivity to local overfitting, worsening client drift. We propose DP-FedAdamW, the first AdamW-based optimizer for DPFL. It restores the effectiveness of AdamW under DP by stabilizing second-moment variance, removing DP-induced bias, and aligning local updates with the global descent direction to curb client drift. Theoretically, we establish an unbiased second-moment estimator and prove a linearly accelerated convergence rate without any heterogeneity assumption, while providing tighter (\varepsilon,\delta)-DP guarantees. Our empirical results demonstrate the strong performance of DP-FedAdamW across language and vision Transformers, as well as ResNet-18. On Tiny-ImageNet (Swin-Base, \varepsilon=1), DP-FedAdamW outperforms the state-of-the-art (SOTA) by 5.83%. The code is available at https://github.com/junkangLiu0/DP-FedAdamW.