Guaranteed scheduling for switches with configuration overhead

• Authors: Brian Towles;William J. Dally
• Venue: IEEE/ACM Transactions on Networking (TON)
• Year: 2003

In this paper, we present three algorithms that provide performance guarantees for scheduling switches, such as optical switches, with configuration overhead. Each algorithm emulates an unconstrained (zero overhead) switch by accumulating a batch of configuration requests and generating a corresponding schedule for a constrained switch. Speedup is required both to cover the configuration overhead of the switch and to compensate for empty slots left by the scheduling algorithm. Scheduling algorithms are characterized by the number of configurations Ns they require to cover a batch of requests and the speedup required to compensate for empty slots Smin. Initially, all switch reconfiguration is assumed to occur simultaneously. We show that a well-known exact matching algorithm, EXACT, leaves no empty slots (i.e., Smin = 1), but requires Ns ≈ N2 configurations for an N-port switch leading to high configuration overhead or large batches and, hence, high delay. We present two new algorithms that reduce the number of configurations required substantially. MIN covers a batch of requests in the minimum possible number of configurations, Ns = N, but at the expense of many empty slots, Smin ≈ 4log2 N. DOUBLE strikes a balance, requiring twice as many configurations,Ns = 2N, while reducing the number of empty slots so that Smin = 2. Loosening the restriction on reconfiguration times, the scheduling problem is cast as an open shop. The best known practical scheduling algorithm for open shops, list scheduling (LIST), gives the same emulation requirements as DOUBLE. Therefore, we conclude that our architecture gains no advantages from allowing arbitrary switch reconfiguration. Finally, we show that DOUBLE and LIST offer the lowest required speedup to emulate an unconstrained switch across a wide range of port count and delay.