Advantage Normalization in MDPs
- Advantage normalization is a procedure that reformulates reward structures in MDPs to preserve invariant advantage relationships across policies.
- It employs a geometric framework that maps actions to affine vectors, enabling reward balancing and zero-value normalization for efficient policy selection.
- This approach facilitates value-free solvers that improve convergence rates and policy improvement robustness in reinforcement learning applications.
Advantage normalization is a procedure in sequential decision-making and reinforcement learning that transforms the reward structure and value function of a Markov Decision Process (MDP) without altering the fundamental advantage relationships of actions under any policy. This approach is motivated by a geometric perspective on MDPs, in which every action is mapped to a vector in an affine space and each policy corresponds to a hyperplane orthogonal to this vector representation. By leveraging an advantage-preserving reward transformation, advantage normalization facilitates the design of algorithms—such as reward balancing solvers—that efficiently compute near-optimal policies, often with improved convergence properties and “value-free” dynamics. In practical terms, advantage normalization is used to simplify policy selection, reduce dependence on value-function computations, and clarify the theoretical connections between MDP geometry, policy improvement, and optimality (Mustafin et al., 9 Jul 2024).
1. Geometric Representation of MDPs
The geometric framework for MDPs begins by embedding the action space into dimensions, with each action represented by a vector , where is the immediate reward and are the transition probabilities. The BeLLMan equation is expressed as:
This formulation is interpreted as an inner product between the action vector and an extended policy vector . When an action is part of the policy , is orthogonal to (as formalized in Proposition 2.1). The set of allowable action vectors for a state is defined by affine constraints:
- for all , and .
A policy is thus geometrically characterized by the hyperplane through the action vectors it selects.
2. Advantage-Preserving Normalization Procedure
The core advantage normalization procedure adjusts the reward of every action in the MDP such that the values of the optimal policy become zero at every state, while leaving the advantage of any action under any policy unchanged. For state and a chosen shift , the transformation modifies rewards as follows:
- For actions with : .
- For actions with : .
This corresponds geometrically to the map , effectively “lifting” the self-loop line by . Applying for all states yields a normalized MDP in which the optimal policy achieves zero value everywhere.
3. Advantage Invariance and Its Consequences
The principal property of the normalization transformation is the invariance of the advantage function:
Under , both rewards and policy values are shifted in such a way that the advantage of every action remains constant. Algebraic derivations in the source establish that post-transformation for all actions and policies. Thus, the critical ordering of actions (for policy improvement and selection) is unaffected, and optimal policy characteristics persist under normalization.
4. Reward Balancing Algorithms
Advantage normalization motivates a category of “reward balancing” algorithms that iteratively adjust action rewards to reach a normal form where the maximum reward at each state is zero. In this normal form, optimal actions correspond exactly to those with zero reward, and all others have strictly negative reward. The Value-Free Solver (VFS) is an explicit example:
- For each state , compute $\delta_s = \text{minimum reward in state$s$} / (\gamma p_s - 1)$.
- Update the MDP via .
- Repeat until the maximum reward in every state is zero (or nearly zero, subject to an approximation lemma).
A proven lemma (Lemma 4.1) bounds the suboptimality by , with being the minimal reward in any state. Thus, reward balancing ensures -optimality without explicit computation of policy value functions.
5. Convergence Analysis and Comparison to Standard Methods
Within the geometric framework, traditional Policy Iteration (PI) and Value Iteration (VI) can be interpreted as sequences of hyperplane constructions approaching the optimal one. PI selects hyperplanes such that selected actions have maximal advantage relative to the current estimate, with improvements driven by “lifting” toward optimality. VI updates state values by:
where is the action maximizing the advantage with respect to . The convergence of reward balancing solvers, particularly in tree-structured MDPs, is demonstrated to be faster than the discount rate , and sample complexity can outperform previous state-of-the-art results in settings with unknown transitions. Some open theoretical questions remain about the equivalence of VFS and standard VI dynamics.
6. Practical Implications and Applications
Advantage normalization clarifies the geometric structure underlying MDP solution methods, simplifying policy selection by reducing the problem to finding actions with zero reward after normalization. The value-free nature of reward balancing solvers eliminates dependence on value-function estimation, mitigating sensitivity to initialization and facilitating rapid convergence in various MDP architectures (especially hierarchical and tree-based). The approach may inform the development of more robust reinforcement learning algorithms and function-approximation methods, particularly where partial knowledge of the MDP prevails. Moreover, the geometric and normalization perspective enhances the understanding of advantage as the fundamental currency of policy improvement, potentially influencing future research in dynamic programming and reinforcement learning (Mustafin et al., 9 Jul 2024).