Multiple priorities in a two-lane buffered crossbar

A significant advantage of buffered crossbar switches is that they can directly operate on variable-size packets. However, in order to support multiple priority levels, separate queues per priority are needed at each crosspoint, in order to prevent HOL blocking and buffer hogging; these queues are expensive because they each need a size of at least one maximum-size packet. In this paper, we propose a scheme that uses only two queues per crosspoint to effectively support multiple priorities. We adaptively adjust the priority levels of the two queues so that most traffic goes through the "lower" queue, while the "upper" queue remains usually available for higher priority packets to overtake the former. Through simulation, and assuming 8 priority levels, we compare our scheme to an ideal system that uses 8 queues per crosspoint. For realistic traffic, the two systems perform almost identically, although ours uses 4 times less memory in the crossbar. Even under a highly irregular traffic pattern, Bursts60, our system does not increase the average delay of any priority level by more than 75% compared to the ideal system.