Preventing tree saturation in multistage networks

Abstract

In a large-scale multiprocessing system, contention for a particular memory location (called hotspot), may create congestion in the interconnection network. Usually a tree of saturated buffers rooted at the hot memory module and extending to the processors is formed which causes excessive delay for both hotspot and regular nonhotspot requests. To prevent tree saturation, a simple combining technique is proposed. The technique attempts to increase the chances of request combining. The new scheme called input queue combining uses simpler queueing structure compared to those proposed for conventional pairwise combining. The component count in each queue reduces to one-third of that in pairwise combining and the number for the waitbuffer reduces to half. While conventional pairwise combining is effective only for smaller networks, the proposed technique eliminates tree saturation even in very large networks, with reasonable delay. Networks delays reported here are among the lowest reported to date. >