Constrained Bi-Level Optimization: Proximal Lagrangian Value function Approach and Hessian-free Algorithm
This work addresses a bottleneck in machine learning applications by providing a more efficient method for solving constrained bi-level optimization problems, though it is incremental as it builds on existing gradient-based approaches.
The paper tackles constrained bi-level optimization problems with coupled constraints, which are computationally intensive due to Hessian matrix calculations, by introducing a Hessian-free algorithm (LV-HBA) that achieves superior practical performance in empirical results.
This paper presents a new approach and algorithm for solving a class of constrained Bi-Level Optimization (BLO) problems in which the lower-level problem involves constraints coupling both upper-level and lower-level variables. Such problems have recently gained significant attention due to their broad applicability in machine learning. However, conventional gradient-based methods unavoidably rely on computationally intensive calculations related to the Hessian matrix. To address this challenge, we begin by devising a smooth proximal Lagrangian value function to handle the constrained lower-level problem. Utilizing this construct, we introduce a single-level reformulation for constrained BLOs that transforms the original BLO problem into an equivalent optimization problem with smooth constraints. Enabled by this reformulation, we develop a Hessian-free gradient-based algorithm-termed proximal Lagrangian Value function-based Hessian-free Bi-level Algorithm (LV-HBA)-that is straightforward to implement in a single loop manner. Consequently, LV-HBA is especially well-suited for machine learning applications. Furthermore, we offer non-asymptotic convergence analysis for LV-HBA, eliminating the need for traditional strong convexity assumptions for the lower-level problem while also being capable of accommodating non-singleton scenarios. Empirical results substantiate the algorithm's superior practical performance.