Optimizing the Numbers of Queries and Replies in Federated Learning with Differential Privacy

TL;DR

This paper analyzes how to optimally set the number of queries and replies in federated learning with differential privacy to maximize model accuracy, using convergence analysis and experiments on MNIST and FEMNIST datasets.

Contribution

It provides a convergence-based method to determine optimal query and reply counts in federated learning with differential privacy, improving model accuracy.

Findings

Properly setting queries and replies enhances accuracy.

Optimal numbers depend on privacy mechanisms and convergence analysis.

Experimental results confirm the theoretical analysis.

Abstract

Federated learning (FL) empowers distributed clients to collaboratively train a shared machine learning model through exchanging parameter information. Despite the fact that FL can protect clients' raw data, malicious users can still crack original data with disclosed parameters. To amend this flaw, differential privacy (DP) is incorporated into FL clients to disturb original parameters, which however can significantly impair the accuracy of the trained model. In this work, we study a crucial question which has been vastly overlooked by existing works: what are the optimal numbers of queries and replies in FL with DP so that the final model accuracy is maximized. In FL, the parameter server (PS) needs to query participating clients for multiple global iterations to complete training. Each client responds a query from the PS by conducting a local iteration. Our work investigates how many times the PS should query clients and how many times each client should reply the PS. We investigate two most extensively used DP mechanisms (i.e., the Laplace mechanism and Gaussian mechanisms). Through conducting convergence rate analysis, we can determine the optimal numbers of queries and replies in FL with DP so that the final model accuracy can be maximized. Finally, extensive experiments are conducted with publicly available datasets: MNIST and FEMNIST, to verify our analysis and the results demonstrate that properly setting the numbers of queries and replies can significantly improve the final model accuracy in FL with DP.