Learning to branch with Tree MDPs
This work addresses the learning-to-branch problem in MILP solvers, which is incremental as it builds on prior research to enhance heuristic optimization.
The paper tackles the problem of learning branching rules for Mixed Integer Linear Program (MILP) solvers by proposing tree Markov Decision Processes (tree MDPs) as a reinforcement learning framework, demonstrating improved learning convergence in computational experiments.
State-of-the-art Mixed Integer Linear Program (MILP) solvers combine systematic tree search with a plethora of hard-coded heuristics, such as the branching rule. The idea of learning branching rules from data has received increasing attention recently, and promising results have been obtained by learning fast approximations of the strong branching expert. In this work, we instead propose to learn branching rules from scratch via Reinforcement Learning (RL). We revisit the work of Etheve et al. (2020) and propose tree Markov Decision Processes, or tree MDPs, a generalization of temporal MDPs that provides a more suitable framework for learning to branch. We derive a tree policy gradient theorem, which exhibits a better credit assignment compared to its temporal counterpart. We demonstrate through computational experiments that tree MDPs improve the learning convergence, and offer a promising framework for tackling the learning-to-branch problem in MILPs.