Stress resistant scheduling algorithms for CIOQ switches

Practical crossbar scheduling algorithms for CIOQ switches such as PIM and i-SLIP can perform poorly under extreme traffic conditions, frequently failing to be work-conserving. The common practice of evaluating crossbar scheduling algorithms according to the packet delay under random admissible traffic tends to obscure significant differences that affect the robustness of different algorithms when exposed to extreme conditions. On the other hand, algorithms such as LOOM with provably good worst-case performance, don't lend themselves readily to high performance implementation. We advocate evaluating crossbar scheduling algorithms using targeted stress tests which seek to probe the performance boundaries of competing alternatives. Appropriately designed stress tests can reveal key differences among algorithms and can provide the insight needed to spur the development of better solutions. In this paper we introduce the use of stress testing for crossbar scheduling and use it to evaluate the performance of PIM, i-SLIP and LOOFA. Our results show that PIM and i-SLIP need large speedups in order to perform well on stress tests, while LOOM can deliver excellent performance, even for speedups less than 1.5. We then develop improved versions of PIM and i-SLIP which take output queue lengths into account, making them much more robust. We also develop an algorithm which closely approximates the behavior (and performance) of LOOFA, but which admits a straightforward, high performance hardware implementation.