SONAR: Benchmarking Topology and Collaboration in Decentralized Learning

Decentralized machine learning relies on peer-to-peer communication, yet the role of network topology in shaping learning dynamics remains poorly understood due to the lack of controlled, reproducible evaluation frameworks. We present \textbf{SONAR}, a modular framework for topology-aware decentralized learning that unifies communication, topology management, and fine-grained telemetry, enabling end-to-end measurement of performance, communication, robustness, and privacy under consistent conditions. Using SONAR, we show that topology is a first-class systems variable whose impact amplifies with scale and data heterogeneity. We find that sparse, structured topologies (e.g., rings and tori) can achieve comparable or superior accuracy to dense graphs at substantially lower communication cost under circumstances, revealing a clear efficiency frontier. We further identify and provide insights on collaborator collapse, a systematic failure mode in adaptive collaboration, where similarity-based neighbor selection reduces diversity and degrades generalization. By exposing topology as a controllable and measurable dimension, SONAR enables systematic, reproducible evaluation of decentralized learning and provides practical guidance for designing efficient and robust collaborative systems.