On the Surprising Effectiveness of a Single Global Merging in Decentralized Learning

TL;DR

This paper demonstrates that a single global merging at the end of decentralized learning can significantly enhance performance, especially under high data heterogeneity, matching the convergence rate of parallel SGD.

Contribution

It provides the first theoretical analysis showing that a final global merging can match parallel SGD convergence rates, challenging previous assumptions about local model discrepancies.

Findings

Concentrating communication in later training stages improves test performance.

A single global merging at the end can significantly boost decentralized learning.

Theoretical analysis explains how global merging matches parallel SGD convergence.

Abstract

Decentralized learning provides a scalable alternative to parameter-server-based training, yet its performance is often hindered by limited peer-to-peer communication. In this paper, we study how communication should be scheduled over time, including determining when and how frequently devices synchronize. Counterintuitive empirical results show that concentrating communication budgets in the later stages of decentralized training remarkably improves global test performance. Surprisingly, we uncover that fully connected communication at the final step, implemented by a single global merging, can significantly improve the performance of decentralized learning under high data heterogeneity. Our theoretical contributions, which explain these phenomena, are the first to establish that the globally merged model of decentralized SGD can match the convergence rate of parallel SGD. Technically, we reinterpret part of the discrepancy among local models, which were previously considered as detrimental noise, as constructive components essential for matching this rate. This work provides evidence that decentralized learning is able to generalize under high data heterogeneity and limited communication, while offering broad new avenues for model merging research.