insight - Federated reinforcement learning - # Federated reinforcement learning in heterogeneous environments

Core Concepts

The core message of this paper is to introduce the Convergence-AwarE SAmpling with scReening (CAESAR) aggregation scheme, which enhances the learning of individual agents across varied Markov Decision Processes (MDPs) in federated reinforcement learning settings characterized by environmental heterogeneity.

Abstract

This paper addresses the challenges of training individual policies with environmental heterogeneity in federated reinforcement learning (FedRL). The authors formulate the problem setup of FedRL in heterogeneous environments, where each agent learns a localized policy for its designated MDP.

The paper explores various aggregation schemes for selecting a subset of agents to aggregate their value functions during the federated update process. The authors introduce the CAESAR aggregation scheme, which combines convergence-aware sampling with a selective screening mechanism.

The convergence-aware sampling step identifies probable "peers" - agents learning in the same MDP - by assessing the convergence trends of their value functions. The selective screening process then refines the selected agents, prioritizing those with superior performance. This dual-layered approach of CAESAR effectively mitigates the risk of suboptimal peer selection, enhancing the learning efficiency of agents in their respective MDPs.

The authors validate the effectiveness of CAESAR through experiments in a custom-built GridWorld environment and the FrozenLake-v1 task, each presenting varying levels of environmental heterogeneity. CAESAR demonstrates robust performance across diverse scenarios, outperforming other aggregation schemes, including the hypothetical "Peers" approach that assumes prior knowledge of agent-MDP assignments.

Key Insights Distilled From

by Hei ... at **arxiv.org** 04-01-2024

Stats

The paper does not contain any explicit numerical data or statistics to support the key logics. The analysis is primarily based on qualitative observations and comparisons of the learning curves of different aggregation schemes.

Quotes

"Existing FedRL methods typically aggregate agents' learning by averaging the value functions across them to improve their performance. However, this aggregation strategy is suboptimal in heterogeneous environments where agents converge to diverse optimal value functions."

"CAESAR is an aggregation strategy used by the server that combines convergence-aware sampling with a screening mechanism. By exploiting the fact that agents learning in identical MDPs are converging to the same optimal value function, CAESAR enables the selective assimilation of knowledge from more proficient counterparts, thereby significantly enhancing the overall learning efficiency."

Deeper Inquiries

In order to extend the CAESAR scheme to handle continuous state and action spaces, several modifications and adaptations would be necessary. In the tabular setting explored in the paper, the Q-values were stored in a table, making it feasible to calculate the similarity between agents based on their Q-values. However, in continuous spaces, this tabular representation is not practical.

One approach to extend CAESAR to continuous spaces would be to utilize function approximation methods such as neural networks to represent the Q-values. Agents could use deep Q-networks (DQN) or other deep reinforcement learning architectures to approximate the Q-values for continuous state and action spaces. The convergence of value functions could then be assessed based on the similarity of the neural network weights or outputs.

Additionally, in continuous spaces, the dissimilarity metric between value functions could be calculated using techniques like mean squared error or other distance metrics suitable for continuous functions. The probabilities for selecting peers could be updated based on the changes in these dissimilarity metrics over time, similar to the approach in the tabular setting.

Furthermore, in continuous spaces, the screening process in CAESAR could involve comparing the performance of agents based on their policy outputs or other performance metrics derived from the neural network approximations. Agents with superior performance could be prioritized for inclusion in the aggregation process.

Overall, extending the CAESAR scheme to handle continuous state and action spaces would involve adapting the convergence detection, dissimilarity metrics, and peer selection processes to suit the characteristics of neural network-based function approximations in continuous environments.

If the agents employ policy-based methods instead of value-based approaches like Q-learning, the performance of the CAESAR scheme would be impacted in several ways.

In a policy-based setting, agents learn a policy directly without explicitly estimating value functions. The convergence of policy-based methods is different from that of value-based methods, as policies can converge to different optimal policies even in the same environment. This difference in convergence behavior would affect the effectiveness of the convergence-aware sampling process in CAESAR.

To adapt CAESAR for policy-based methods, the convergence detection mechanism would need to be redefined to assess the similarity of learned policies among agents. This could involve comparing the actions chosen by agents in similar states or evaluating the policy outputs directly to determine convergence.

The screening process in CAESAR, which prioritizes agents based on their performance, would also need to be adjusted for policy-based methods. Performance metrics in a policy-based setting could include measures of policy stability, exploration efficiency, or other relevant indicators of policy quality.

Overall, the performance of CAESAR in a policy-based setting would depend on the effectiveness of the convergence detection and screening mechanisms tailored to policy learning. Adapting CAESAR for policy-based methods would require redefining the convergence-aware sampling and screening processes to align with the characteristics of policy optimization.

Yes, the principles of the CAESAR scheme can be applied to other federated learning settings beyond reinforcement learning, such as supervised or unsupervised learning tasks. The core idea of CAESAR, which involves convergence-aware sampling and selective screening for knowledge aggregation, can be generalized to various machine learning tasks that involve distributed learning across multiple agents.

In supervised learning tasks, where agents aim to learn a mapping from input to output data, CAESAR's convergence-aware sampling mechanism can be adapted to assess the similarity of learned models or parameters across agents. The selective screening process can prioritize agents with superior performance based on metrics like accuracy or loss functions.

Similarly, in unsupervised learning tasks, where agents aim to discover patterns or structures in data without explicit labels, CAESAR's principles can be applied to detect convergence of learned representations or clustering structures. The screening process can focus on agents that exhibit better clustering or representation learning capabilities.

The key in applying CAESAR to other federated learning settings lies in customizing the convergence detection and screening mechanisms to suit the specific characteristics and objectives of the learning task. By adapting the principles of CAESAR to supervised or unsupervised learning scenarios, the scheme can enhance collaborative learning efficiency and performance across diverse environments and tasks.

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