Motivated by the use of high speed circuit switches in large scale data centers, we consider the problem of circuit switch scheduling. In this problem we are given demands between pairs of servers and the goal is to schedule at every time step a matching between the servers while maximizing the total satisfied demand over time. The crux of this scheduling problem is that once one shifts from one matching to a different one a fixed delay @math is incurred during which no data can be transmitted. For the offline version of the problem we present a @math approximation ratio (for any constant @math ). Since the natural linear programming relaxation for the problem has an unbounded integrality gap, we adopt a hybrid approach that combines the combinatorial greedy with randomized rounding of a different suitable linear program. For the online version of the problem we present a (bi-criteria) @math -competitive ratio (for any constant @math ) that exceeds time by an additive factor of @math . We note that no uni-criteria online algorithm is possible. Surprisingly, we obtain the result by reducing the online version to the offline one.
Hybrid circuit and packet switching for data center networking (DCN) has received considerable research attention recently. A hybrid-switched DCN employs a much faster circuit switch that is reconfigurable with a nontrivial cost, and a much slower packet switch that is reconfigurable with no cost, to interconnect its racks of servers. The research problem is, given a traffic demand matrix (between the racks), how to compute a good circuit switch configuration schedule so that the vast majority of the traffic demand is removed by the circuit switch, leaving a remaining demand matrix that contains only small elements for the packet switch to handle. In this paper, we propose two new hybrid switch scheduling algorithms under two different scheduling constraints. Our first algorithm, called 2-hop Eclipse, strikes a much better tradeoff between the resulting performance (of the hybrid switch) and the computational complexity (of the algorithm) than the state of the art solution Eclipse Eclipse++. Our second algorithm, called BFF (best first fit), is the first hybrid switching solution that exploits the potential partial reconfiguration capability of the circuit switch for performance gains. Increasingly, proposals for new datacenter networking fabrics employ some form of traffic scheduling---often to avoid congestion, mitigate queuing delays, or avoid timeouts. Fundamentally, practical implementations require estimating upcoming traffic demand. Unfortunately, as our results show, it is difficult to accurately predict demand in typical datacenter applications more than a few milliseconds ahead of time. We explore the impact of errors in demand estimation on traffic scheduling in circuit-switched networks. We show that even relatively small estimation errors such as shifting the arrival time of at most 30 of traffic by a few milliseconds can lead to suboptimal schedules that dramatically reduce network efficiency. Existing systems cope by provisioning extra capacity---either on each circuit, or through the addition of a separate packet-switched fabric. We show through simulation that indirect traffic routing is a powerful technique for recovering from the inefficiencies of suboptimal scheduling under common datacenter workloads, performing as well as networks with 16 extra circuit bandwidth or a packet switch with 6 of the circuit bandwidth. 