Ensemble Distillation for Robust Model Fusion in Federated Learning (2006.07242v3)

Abstract: Federated Learning (FL) is a machine learning setting where many devices collaboratively train a machine learning model while keeping the training data decentralized. In most of the current training schemes the central model is refined by averaging the parameters of the server model and the updated parameters from the client side. However, directly averaging model parameters is only possible if all models have the same structure and size, which could be a restrictive constraint in many scenarios. In this work we investigate more powerful and more flexible aggregation schemes for FL. Specifically, we propose ensemble distillation for model fusion, i.e. training the central classifier through unlabeled data on the outputs of the models from the clients. This knowledge distillation technique mitigates privacy risk and cost to the same extent as the baseline FL algorithms, but allows flexible aggregation over heterogeneous client models that can differ e.g. in size, numerical precision or structure. We show in extensive empirical experiments on various CV/NLP datasets (CIFAR-10/100, ImageNet, AG News, SST2) and settings (heterogeneous models/data) that the server model can be trained much faster, requiring fewer communication rounds than any existing FL technique so far.

• The paper introduces FedDF, an ensemble distillation approach that fuses heterogeneous client models using unlabeled data.
• The method significantly reduces communication rounds and outperforms FedAvg on non-i.i.d. datasets.
• Extensive experiments on CV and NLP tasks confirm FedDF’s robustness, flexibility, and real-world applicability.

Ensemble Distillation for Robust Model Fusion in Federated Learning: An Insightful Overview

Ensemble Distillation for Robust Model Fusion in Federated Learning, addresses the challenges associated with Federated Learning (FL), particularly focusing on the aggregation of heterogeneous client models. The paper presents an innovative approach, termed FedDF (Federated Distillation Fusion), leveraging ensemble distillation to improve model fusion in FL settings. The proposed method effectively mitigates the inherent limitations of parameter averaging methods such as FedAvg, especially in scenarios involving non-i.i.d. data distributions and heterogeneous model architectures.

Core Contributions

Ensemble Distillation for Federated Learning:

The paper introduces an ensemble distillation technique enabling the server to aggregate knowledge from heterogeneous client models. This approach is significant as it allows the integration of client models that may differ in architecture, size, and numerical precision. The technique involves using unlabeled data to distill knowledge from the ensemble of client models into a single central model.

Flexibility and Robustness:

FedDF provides flexibility in handling client models with varying architectures and training data distributions. The use of unlabeled data for distillation, be it datasets from other domains or synthetic data generated by GANs, ensures that the model remains robust against privacy risks while maintaining high performance.

Empirical Validation:

The method's efficacy is validated through extensive experiments on several CV and NLP datasets (CIFAR-10/100, ImageNet, AG News, SST2) and settings. The results show that FedDF can achieve higher accuracy with fewer communication rounds compared to traditional FL methods such as FedAvg and its extensions (FedProx, FedAvgM).

Numerical Results and Claims

Reduction in Communication Rounds:

FedDF demonstrates a substantial reduction in the number of communication rounds needed to reach target accuracy levels. For instance, in a setup involving ResNet-8 on CIFAR-10 with 40 epochs of local training per round, FedDF required approximately 20 rounds to achieve 80% accuracy, whereas FedAvg needed up to 100 rounds.

Performance with Non-I.I.D. Data:

The proposed method exhibits robust performance even with highly heterogeneous data distributions. For example, with a non-i.i.d. degree ($\alpha = 0.1$), FedDF achieved 71.36% accuracy on CIFAR-10, significantly outperforming FedAvg, which struggled to surpass 62.22% accuracy in similar conditions.

Impact of Normalization Techniques:

The paper also presents an analysis of the impact of different normalization techniques, highlighting that FedDF is less affected by the non-i.i.d. data issue compared to FedAvg. FedDF’s compatibility with Batch Normalization (BN) stands out, avoiding the need for additional modifications like Group Normalization (GN).

Implications and Future Developments

Theoretical Insights:

The theoretical framework provided in the paper offers a generalization bound for the ensemble performance, which underscores the importance of distribution discrepancies among client data and the fusion efficiency of the distillation dataset. This bound suggests that ensemble diversity positively correlates with model fusion quality, guiding future research toward optimizing ensemble composition.

Real-World Applications:

FedDF’s ability to handle heterogeneous models and data distribution makes it particularly valuable for real-world FL applications involving edge devices with varying capabilities. Scenarios like federated learning on IoT devices, where models may need to be quantized for resource efficiency, can directly benefit from this approach.

Potential Extensions:

Future work could explore enhancements like privacy-preserving extensions, differential privacy, and hierarchical model fusion to further safeguard client data. Moreover, integrating decentralized fusion techniques could lead to more robust FL frameworks against adversarial attacks.

Compatibility with Existing Techniques:

The compatibility of FedDF with other FL techniques, such as local regularization or momentum-based updates, can lead to its adoption in broader contexts, providing a comprehensive solution in federated machine learning deployments.

Conclusion

The paper presents a profound advancement in FL technologies through the introduction of FedDF, which effectively addresses key challenges in model fusion within federated learning. The empirical validations, robust theoretical foundations, and practical implications highlight the potential for FedDF to significantly improve the performance and efficiency of federated learning systems, particularly in heterogeneous and privacy-sensitive environments.