Towards Straggler-Resilient Split Federated Learning: An Unbalanced Update Approach

Rui Li (Rochester Institute of Technology) · Zhe Li (Rochester Institute of Technology) · Haibo Yang (Rochester Institute of Technology) · Dandan Liang (Rochester Institute of Technology) · Jianing Zhang (Purdue University) · Evan Chen (Purdue University)

adaptive tuningcommunication roundcomputational offloadingconvergence ratedistributed learning systemsmodel updatescalabilitysplit federated learningsplit serverstraggler issuestraggler-resilientsynchronization requirementtime latencyunbalanced update mechanism

Split Federated Learning (SFL) enables scalable training on edge devices by combining the parallelism of Federated Learning (FL) with the computational offloading of Split Learning (SL). Despite its great success, SFL suffers significantly from the well-known straggler issue in distributed learning systems. This problem is exacerbated by the dependency between Split Server and clients: the Split Server side model update relies on receiving activations from clients. Such synchronization requirement introduces significant time latency, making straggler a critical bottleneck to the scalability and efficiency of the system. To mitigate this problem, we propose *MU-SplitFed*, a straggler-resilient SFL algorithm that decouples training progress from straggler delays via a simple yet effective unbalanced update mechanism. By enabling the server to perform $\tau$ local updates per client round, *MU-SplitFed* achieves convergence rate $\mathcal{O}(\sqrt{d/(\tau T)})$, showing a linear reduction in communication round by a factor of $\tau$. Experiments demonstrate that *MU-SplitFed* consistently outperforms baseline methods with the presence of stragglers and effectively mitigates their impact through adaptive tuning of $\tau$.