Congestion Control for High Performance Virtual Cut-through Networks

L. Fernández and J.M. Garcia (Spain)


Network congestion control, highperformance networks, virtual cutthrough


Network performance in multiprocessors degrades dramat ically when the network is beyond saturation. Virtual chan nels and adaptive routing soften this effect, but they can not eliminate it. Moreover, close to saturation, packets be come blocked faster than deadlock avoidance or recover ing mechanisms can manage. In the latter case, the dead lock frequency becomes so high that it makes recovery so lutions inacceptable. Network congestion effects include performance breakdown, exponential augment of packet la tency and asymmetries in the use of buffers in symmetric networks, making the network inappropriate for providing QoS. Congestion control has been widely discussed in lit erature in wormhole contexts. However, due to the ease of integrating large buffers in switches, cut-through is becom ing popular, opening a new branch of study. To be able to deal with network congestion some global network information would be necessary. However, for propagating such information, additional resources must be used. In this paper, a congestion control technique for k-ary n-cubes with virtual cut-through is proposed. This technique uses local information for congestion detection, provided by the number of occupied buffers, to estimate the congestion level. When congestion is detected, it acts by limiting the injection of new messages in the source of the problem. This source is deduced by looking into the headers of the buffered and recently routed messages.

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