A Hierarchical Gradient Tracking Algorithm for Mitigating Subnet-Drift in Fog Learning Networks

Evan Chen; Shiqiang Wang; Christopher G. Brinton

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arXiv · 2409.17430

A Hierarchical Gradient Tracking Algorithm for Mitigating Subnet-Drift in Fog Learning Networks

Evan Chen·Shiqiang Wang·Christopher G. Brinton

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Abstract

Federated learning (FL) encounters scalability challenges when implemented over fog networks that do not follow FL's conventional star topology architecture. Semi-decentralized FL (SD-FL) has proposed a solution for device-to-device (D2D) enabled networks that divides model cooperation into two stages: at the lower stage, D2D communications is employed for local model aggregations within subnetworks (subnets), while the upper stage handles device-server (DS) communications for global model aggregations. However, existing SD-FL schemes are based on gradient diversity assumptions that become performance bottlenecks as data distributions become more heterogeneous. In this work, we develop semi-decentralized gradient tracking (SD-GT), the first SD-FL methodology that removes the need for such assumptions by incorporating tracking terms into device updates for each communication layer. Our analytical characterization of SD-GT reveals upper bounds on convergence for non-convex, convex, and strongly-convex problems. We show how the bounds enable the development of an optimization algorithm that navigates the performance-efficiency trade-off by tuning subnet sampling rate and D2D rounds for each global training interval. Our subsequent numerical evaluations demonstrate that SD-GT obtains substantial improvements in trained model quality and communication cost relative to baselines in SD-FL and gradient tracking on several datasets.

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BibTeX RIS

Evan Chen, Shiqiang Wang, Christopher G. Brinton. 2024-09-25. A Hierarchical Gradient Tracking Algorithm for Mitigating Subnet-Drift in Fog Learning Networks. https://arxiv.org/abs/2409.17430

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