Fair, efficient and scalable scheduling without per-flow state

In recent years, parallel computer systems are being increasingly used in multi-user environments, with several users sharing the interconnection network at the same time. As a result a large number frequently contend for link bandwidth at the core switches in the network. Traditional fair scheduling disciplines need to maintain per-flow states and perform packet scheduling on a per-flow basis, which increases the complexity of implementation at high speeds for large numbers of flows. We present an efficient, fair, simple and scalable solution which requires the schedulers in the entire network to maintain only a per-link state as opposed to a per-flow state. In our scheme, the edge switches in the network employ a modified version of the deficit round robin (DRR) scheduler, whereas the core switches use the proposed aggregated flow fair queueing (AFFQ) scheduling discipline. We prove that AFFQ maintains the same relative fairness bound as deficit round robin and has a work complexity of O(1). Our scheme is also applicable in other networks like the Internet, where the number of active flows in the core routers can be very large