Scheduling requests on multi-stage multi-server to increase quality of service

In this paper, we consider the problem of scheduling on a multi-stage multi-server architecture in computer centers to enhance quality of service through minimizing the average completion time of a set of requests. This problem is known to be an NP-hard combinatorial problem. We show that the problem can modeled as a flexible flowshop problem for which few heuristics exist in the flowshop scheduling literature. We propose a new heuristic for this problem that consists of three phases. Simulated Annealing is used in the first phase of the heuristic followed by a greedy algorithm phase. The last phase is a local pair-wise exchange procedure. An extensive computational experiment has been conducted to compare the existing heuristics with each other for the first time. Moreover, the proposed heuristic is compared with the existing heuristics. The results indicate that the proposed heuristic significantly outperforms the existing ones.