FedRS: Federated Learning with Restricted Softmax for Label Distribution Non-IID Data

Federated Learning (FL) aims to generate a global shared model via collaborating decentralized clients with privacy considerations. Unlike standard distributed optimization, FL takes multiple optimization steps on local clients and then aggregates the model updates via a parameter server. Although this significantly reduces communication costs, the non-iid property across heterogeneous devices could make the local update diverge a lot, posing a fundamental challenge to aggregation. In this paper, we focus on a special kind of non-iid scene, i.e., label distribution skew, where each client can only access a partial set of the whole class set. Considering top layers of neural networks are more task-specific, we advocate that the last classification layer is more vulnerable to the shift of label distribution. Hence, we in-depth study the classifier layer and point out that the standard softmax will encounter several problems caused by missing classes. As an alternative, we propose "Restricted Softmax" to limit the update of missing classes' weights during the local procedure. Our proposed FedRS is very easy to implement with only a few lines of code. We investigate our methods on both public datasets and a real-world service awareness application. Abundant experimental results verify the superiorities of our methods.