Tree-Saturation Control in the AC Velocity Cluster Interconnect

In a multi-user production cluster there is no control over the intra-cluster communication patterns, which can cause unanticipated hot spots to occur in the cluster interconnect. In a multistage interconnect a common side effect of such a hot-spot is the roll-over of the saturation to other areas in the interconnect that were otherwise not in the direct path of the primary congested element. This paper investigates the effects of treesaturation in the interconnect of the AC Velocity cluster, which is a multistage interconnect constructed out of 40 GigaNet switches. The main congestion control mechanism employed at the GigaNet switches is a direct feedback to the traffic source, allowing for fast control over the source of the congestion, avoiding the spread from the congestion area. The experiments reported are designed to examine the effects of the congestion control in detail.