Federated learning (FL) is a privacy-preserving paradigm for collaboratively training a global model from decentralized clients. However, the performance of FL is hindered by non-independent and identically distributed (non-IID) data and device heterogeneity. In this work, we revisit this key challenge through the lens of gradient conflicts on the server side. Specifically, we first investigate the gradient conflict phenomenon among multiple clients and reveal that stronger heterogeneity leads to more severe gradient conflicts. To tackle this issue, we propose FedGH, a simple yet effective method that mitigates local drifts through Gradient Harmonization. This technique projects one gradient vector onto the orthogonal plane of the other within conflicting client pairs. Extensive experiments demonstrate that FedGH consistently enhances multiple state-of-the-art FL baselines across diverse benchmarks and non-IID scenarios. Notably, FedGH yields more significant improvements in scenarios with stronger heterogeneity. As a plug-and-play module, FedGH can be seamlessly integrated into any FL framework without requiring hyperparameter tuning.

该论文通过梯度冲突现象研究了联邦学习中来自多个客户端的异构性问题，并提出了一种简单有效的方法FedGH，通过梯度协调来解决此问题。实验证明，FedGH在不同基准和非IID场景中能显著提升现有的联邦学习方法，尤其在存在更强异构性的情况下效果更为显著，而且它可以方便地集成到任何联邦学习框架中，无需超参数调整。

通过梯度协调解决异构联邦学习中的非IID问题