Stochastic Semi-Gradient Descent for Learning Mean Field Games with Population-Aware Function Approximation

TL;DR

This paper introduces a novel stochastic gradient descent method, SemiSGD, for learning mean field games by treating policy and population as a unified parameter, enabling simultaneous updates and improved stability.

Contribution

It proposes the first population-aware linear function approximation for MFGs and provides finite-time convergence analysis for the method.

Findings

SemiSGD converges to MFG equilibrium under certain conditions.

PA-LFA effectively handles continuous state-action spaces.

Experimental results validate theoretical convergence and approximation properties.

Abstract

Mean field games (MFGs) model interactions in large-population multi-agent systems through population distributions. Traditional learning methods for MFGs are based on fixed-point iteration (FPI), where policy updates and induced population distributions are computed separately and sequentially. However, FPI-type methods may suffer from inefficiency and instability due to potential oscillations caused by this forward-backward procedure. In this work, we propose a novel perspective that treats the policy and population as a unified parameter controlling the game dynamics. By applying stochastic parameter approximation to this unified parameter, we develop SemiSGD, a simple stochastic gradient descent (SGD)-type method, where an agent updates its policy and population estimates simultaneously and fully asynchronously. Building on this perspective, we further apply linear function approximation (LFA) to the unified parameter, resulting in the first population-aware LFA (PA-LFA) for learning MFGs on continuous state-action spaces. A comprehensive finite-time convergence analysis is provided for SemiSGD with PA-LFA, including its convergence to the equilibrium for linear MFGs -- a class of MFGs with a linear structure concerning the population -- under the standard contractivity condition, and to a neighborhood of the equilibrium under a more practical condition. We also characterize the approximation error for non-linear MFGs. We validate our theoretical findings with six experiments on three MFGs.