Balanced Sparse Tree: A Scalable Network Topology for Large Language Models

The development of large language models (LLMs) has catalyzed unprecedented demand on the computing network, specifically for large-scale, few-hops, and low-latency, which directly underpin LLM task efficiency. However, mainstream topologies such as Clos suffer from costs and latency, while topologies with good scalability have symmetric or collective communication issues. In order to achieve a favorable balance among design metrics, we propose a novel topology named the Balanced Sparse Tree (BST), which is a topology characterized by symmetric design and sparse connections, motivated by hypergraph theory and Steiner Systems. Its degree-diameter upper-bound approaches the Moore Bound for Bipartite Biregular graphs, larger than other known dia-meter-2 topologies. Furthermore, we incorporate differentiated routing, deadlock freedom, and topology-affined deployment into BST. Testbed experiments, simulations, together with modeling analysis, demonstrate the superiority of BST over the state-of-the-art in network scale, latency, bandwidth, and cost. With equivalent scales, BST outperforms Clos with a 50% cost reduction while maintaining comparable performance for AI workloads. Furthermore, BST delivers a 3.9%–11.8% gain in collective communications and has 13.4% improvement over state-of-the-art topologies.