Last-Iterate Global Convergence of Policy Gradients for Constrained Reinforcement Learning

Constrained Reinforcement Learning (CRL) tackles sequential decision-making problems where agents are required to achieve goals by maximizing the expected return while meeting domain-specific constraints, which are often formulated as expected costs. In this setting, policy-based methods are widely used since they come with several advantages when dealing with continuous-control problems. These methods search in the policy space with an action-based or parameter-based exploration strategy, depending on whether they learn directly the parameters of a stochastic policy or those of a stochastic hyperpolicy. In this paper, we propose a general framework for addressing CRL problems via gradient-based primal-dual algorithms, relying on an alternate ascent/descent scheme with dual-variable regularization. We introduce an exploration-agnostic algorithm, called C-PG, which exhibits global last-iterate convergence guarantees under (weak) gradient domination assumptions, improving and generalizing existing results. Then, we design C-PGAE and C-PGPE, the action-based and the parameter-based versions of C-PG, respectively, and we illustrate how they naturally extend to constraints defined in terms of risk measures over the costs, as it is often requested in safety-critical scenarios. Finally, we numerically validate our algorithms on constrained control problems, and compare them with state-of-the-art baselines, demonstrating their effectiveness.

Paper

Similar papers

Peer review

Reviewer zSqV4/10 · confidence 4/52024-06-12

Summary

The paper targets the constrained RL problem and provides a primal-dual method C-PG to solve it. The proposed method is extended to C-PGAE and C-PGPE to handle the constraint cases with risk measures. The paper provides a theoretical analysis of the global last-iterate convergence guarantees toward C-PG and empirically tests the C-PGAE and C-PGPE.

Strengths

The paper is well-written. All the assumptions are standard in literatures or justified. The theoretical result is rigorous.

Weaknesses

The main weakness of the paper comes from the technical novelty. From the algorithm design, the primal-dual method is widely used in constrained optimization problems and constrained RL problems, and the regularization term is also not new in constrained optimization problems [1]. The proposed algorithm basically follows the existing methods without a significant novel design. The algorithm can be naturally adapted to C-PGAE and C-PGPE to handle the constraint cases with risk measures due to the well-established policy gradient for the case. From the theoretical analysis, assumptions 1,2, 3, and 4 are standard assumptions for constrained optimization problems. With these assumptions, the constrained RL problem is transferred to a pure constrained optimization problem. Although the assumptions are justified in RL, I believe there are lots of theoretical results for constrained optimization problems with these assumptions, and can be directly used in constrained RL. Therefore, the theoretical novelty may also be marginal. Although the paper claims that it addresses some theoretical limitations of previous works, these limitations are somehow avoided, rather than being addressed. For example, the paper does not require the softmax policy, but assumption 2 is verified under the softmax policy. The provided convergence rates do not depend on the problem dimension, but the problem dimension may be included in the constants of assumptions 2, 3, and 4. The experiment scenarios are relatively simple, but it is acceptable if the theoretical result is sufficiently solid. [1] Khuzani, Masoud Badiei, and Na Li. "Distributed regularized primal-dual method: Convergence analysis and trade-offs."

Questions

As mentioned in Weaknesses, as the dimension-free property of the proposed algorithm is claimed many times in the paper, have you proven that the problem dimension is independent of the constants of assumptions 2, 3, and 4? What is the reason and the theoretical intuition that it is dimension-free? How does the convergence rate change after introducing the regularization term for the last-iteration convergence? As it introduces an extra error term of $w$ in Theorem 3.1, it may deteriorate the convergence rate. I am willing to increase my score if weaknesses and questions are addressed.

Rating

4

Confidence

4

Soundness

3

Presentation

3

Contribution

2

Limitations

No limitations of the paper are explicitly stated by the author.

Reviewer Vs5i6/10 · confidence 4/52024-07-01

Summary

The paper studies policy-based methods in constrained RL. The author first establishes the last-iterate convergence of the algorithm C-PG under a form a gradient domination assumptions. Then, the author further designs action-based and parameter-based versions of C-PG to handle constraints defined in terms of risk measures over the costs. Finally, the proposed algorithms are validated by numerical examples of control problems.

Strengths

* Despite not being the first paper to establish the last-iterate convergence of primal-dual type algorithms, I believe the convergence results for C-PG is also a meaningful contribution to the literature. * More general constrained RL formulations are studied in the paper, where constrains are defined in terms of risk measures. The author adapts C-PG to another two sample-based variants for these formulations. * Overall, the paper is well-written and easy to follow. The author uses highlights for the two different algorithms/settings, making them easy to distinguish.

Weaknesses

* The role of Section 4 in the paper is unclear to me. Good theoretical results are established for Section 3, yet, there is no general theoretical results for Section 4, besides some cases where results from Section 3 can be directly applied. This makes Section 4 looks like an ''add-on'' to Section 3. * For theoretical results, e.g., Theorems 3.1 and 3.2, it would be better for the author to briefly discuss the proof idea in the main paper. Minor comments: it would be better to make figure legends exactly align with the names of algorithms, e.g., change CPGAE to C-PGAE.

Questions

* I hope the author could further clarify the contribution/role of Section 4 in the paper. * Could the author also discusses how the techniques used in the paper different from that in Ding et al., 2024?

Rating

6

Confidence

4

Soundness

3

Presentation

3

Contribution

3

Limitations

The paper has stated assumptions clearly in the paper. Yet, although the author mentions "The limitations of the work emerges in the final section of the work", I failed to find that section in the paper.

Reviewer ZDP85/10 · confidence 4/52024-07-07

Summary

This paper proposes a general framework for addressing safe RL problems via gradient-based primal-dual algorithms. The authors show that the proposed algorithm exhibit global last-iterate convergence guarantees under gradient domination assumptions. Additionally, the authors validate their algorithms on several constrained control problems.

Strengths

1. This paper is well-written and easy to follow. 2. The paper is technically sound with most claims supported sufficiently. 3. The theoretical analysis seems novel.

Weaknesses

Quality: 1. In Theorem 3.1, it requires w=O(\epsilon) to enforce an overall \epsilon error, but w should be a fixed number in C-PG algorithm. 2. It is better to have some experimental results about cost constraints on MuJoCo and include more baselines, e.g., [Zhang et al., 2020, First Order Constrained Optimization in Policy Space]. Clarity: 1. The range of reward value is [-1, 0], which is a bit weird compared to normal settings. 2. The advantage of parameter-based hyperpolicy is not clearly mentioned in the paper. Significance: One relevant literature is missing. In [Liu et al. 2021, Policy Optimization for Constrained MDPs with Provable Fast Global Convergence], a fast convergence \tilde{O}(1/\epsilon) result is proved without multiple assumptions in this paper.

Questions

Please see the details in "weakness".

Rating

5

Confidence

4

Soundness

2

Presentation

3

Contribution

2

Limitations

There is no potential negative social impact of this work.

Reviewer AKt37/10 · confidence 1/52024-07-10

Summary

This paper studies the problem of constrained MDP. To solve this problem, this paper adopts the policy gradient methods, and specifically they considered the action-based policy gradient method and parameter-based policy gradient method. The algorithm proposed in this paper is a type of primal-dual method. Under certain assumptions, this algorithm is shown to have last iterate convergence. This paper also executes numerical experiments on various environment with their algorithm, and the experiments validate the results in this paper.

Strengths

This paper is a well-written paper. The description of problem setup, theorems, assumptions are clear. Even though this is mainly a theory paper, there are numerical experiments which validates the theoretical results. This paper has results on the last iterate convergence, which is a property not possessed by most of stochastic optimization algorithms.

Weaknesses

I don't see significant weaknesses in this paper.

Questions

Do you have lower bounds showing that these rates are tight?

Rating

7

Confidence

1

Soundness

3

Presentation

3

Contribution

3

Limitations

Yes. The authors addressed all the limitations listed in the guidelines.

Reviewer Vs5i2024-08-09

I thank the reviewer for the explanations. I will maintain my score.

Reviewer ZDP82024-08-13

Thank you for your clarification! I have increased the score.

Reviewer zSqV2024-08-13

Thank you for your detailed responses. The main contribution of the paper is the derivation of theoretical results that simultaneously offer several advantages (i)-(vi). However, some of these advantages ((i)(ii)(iv)(vi)) have been addressed by previous works. A concern is that assumptions like gradient domination, regularity of the objective function, and the existence of saddle points are quite similar to those in standard (non-convex) optimization, which has been extensively studied in the field of optimization. Overall, I think the theoretical results are solid, but the experiments and comparisons with baselines are relatively simple. As I am not fully familiar with the theory bar of this conference, I will finalize my score after discussing it with the AC. If the paper meets the requirements, I am inclined to vote for acceptance.

Program Chairsdecision2024-09-25

Decision

Accept (poster)

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