Online multi-server dial-a-ride problems

Abstract. In an online dial-a-ride problem, a crew of servers has to process transportation requests as they arrive in real time. Possible ob-jective functions include minimizing the makespan and minimizing the sum of completion times. We give competitive algorithms and negative results for several online dial-a-ride problems with multiple servers. Sur-prisingly, in some cases the competitive ratio is dramatically better than that of the corresponding single server problem.

Non-abusiveness Helps:An O(1)-Competitive Algorithm for Minimizing the Maximum Flow Time in the Online Traveling Salesman problem

In the online traveling salesman problem OlTsp requests for visits to cities arrive online while the salesman is traveling. We study the F max-OlTsp where the objective is to minimize the maximum flow time. This objective is particularly interesting for applications. Unfortunately, there can be no competitive algorithm, neither deterministic nor randomized. Hence, competitive analysis fails to distinguish online algorithms. Not even resource augmentation which is helpful in scheduling works as a remedy. This unsatisfactory situation motivates the search for alternative analysis methods. We introduce a natural restriction on the adversary for the F max-OlTsp on the real line. A non-abusive adversary may only move in a direction if there are yet unserved requests on this side. Our main result is an algorithm which achieves a constant competitive ratio against the nonabusive adversary