A unified momentum-based paradigm of decentralized SGD for non-convex models and heterogeneous data

Haizhou Du, Chaoqian Cheng, Chengdong Ni

Abstract: Emerging distributed applications recently boosted the development of decentralized machine learning, especially in IoT and edge computing fields. In real-world scenarios, the common problems of non-convexity and data heterogeneity result in inefficiency, performance degradation, and development stagnation. The bulk of studies concentrate on one of the issues mentioned above without having a more general framework that has been proven optimal. To this end, we propose a unified paradigm called UMP, which comprises two algorithms D-SUM and GT-DSUM based on the momentum technique with decentralized stochastic gradient descent (SGD). The former provides a convergence guarantee for general non-convex objectives, while the latter is extended by introducing gradient tracking, which estimates the global optimization direction to mitigate data heterogeneity (i.e., distribution drift). We can cover most momentum-based variants based on the classical heavy ball or Nesterov's acceleration with different parameters in UMP. In theory, we rigorously provide the convergence analysis of these two approaches for non-convex objectives and conduct extensive experiments, demonstrating a significant improvement in model accuracy up to 57.6% compared to other methods in practice.