On practical stable packet scheduling for bufferless three-stage Clos-network switches

In this paper, we extend our previous work of StablePlus, a stable scheduling algorithm for single-stage packet switches, to bufferless three-stage Clos-network switches. StablePlus is based on an existing stable distributed scheduling algorithm, called DISQUO. We further improve the switching performance by incorporating a heuristic scheduling algorithm after the DISQUO scheduling. In a three-stage Clos-network switch, DISQUO is first used to solve the output contention which generates a stable matching between the input and output ports, then Karol´s algorithm is used to find the feasible internal paths for the matched input and output pairs. However, the latter requires multiple mini-cycles to complete the path-finding task. Worse is that the number of mini-cycles increases as the switch size does, limiting the Clos-network to a small implementable size. By replacing the Hamiltonian Walk in DISQUO with time-division multiplexing (TDM) scheme, we show that the number of required mini-cycles for Karol´s algorithm can be reduced to only two, independent of the switch size. Moreover, with the help of a parallel hardware approach, we can implement packet scheduling in O(1) time complexity. To support high data rates, e.g., 100 Gbps, we can also make the scheduling work on a frame basis. We prove that StablePlus can achieve 100% throughput under any admissible traffic, and by simulations we show that it also has good delay performance.