Providing flow based performance guarantees for buffered crossbar switches

Buffered crossbar switches are a special type of combined input-output queued switches with each crosspoint of the crossbar having small on-chip buffers. The introduction of crosspoint buffers greatly simplifies the scheduling process of buffered crossbar switches, and furthermore enables buffered crossbar switches with speedup of two to easily provide port based performance guarantees. However, recent research results have indicated that, in order to provide flow based performance guarantees, buffered crossbar switches have to either increase the speedup of the crossbar to three or greatly increase the total number of cross- point buffers, both adding significant hardware complexity. In this paper, we present scheduling algorithms for buffered crossbar switches to achieve flow based performance guarantees with speedup of two and with only one or two buffers at each crosspoint. When there is no crosspoint blocking in a specific time slot, only the simple and distributed input scheduling and output scheduling are necessary. Otherwise, the special urgent matching is introduced to guarantee the on-time delivery of crosspoint blocked cells. With the proposed algorithms, buffered crossbar switches can provide flow based performance guarantees by emulating push- in-first-out output queued switches, and we use the counting method to formally prove the perfect emulation. For the special urgent matching, we present sequential and parallel matching algorithms. Both algorithms converge with N iterations in the worst case, and the latter needs less iterations in the average case. Finally, we discuss an alternative backup-buffer implementation scheme to the bypass path, and compare our algorithms with existing algorithms in the literature.