We consider the following general scheduling problem. The input consists of n jobs, each with an arbitrary release time, size, and monotone function specifying the cost incurred when the job is completed at a particular time. The objective is to find a preemptive schedule of minimum aggregate cost. This problem formulation is general enough to include many natural scheduling objectives, such as total weighted flow time, total weighted tardiness, and sum of flow time squared. We give an O(log log P ) approximation for this problem, where P is the ratio of the maximum to minimum job size. We also give an O(1) approximation in the special case of identical release times. These results are obtained by reducing the scheduling problem to a geometric capacitated set cover problem in two dimensions.
Introduction.We consider the following general offline scheduling problem. General scheduling problem (GSP). The input consists of a collection of n jobs, and for each job j there is a positive integer release time r j , a positive integer size p j and a nondecreasing cost (or weight) function w j (t) ≥ 0 specifying a nonnegative cost for each time t > r j . (We will specify later how these weight functions are represented.) A feasible solution is a preemptive schedule, which assigns to each job j time slots [t, t + 1] (not necessarily consecutive and satisfying t ≥ r j ), during which j is run. A job is completed once it has been run for p j units of time. If job j completes at time t, then a cost of w j (t) is incurred for that job. The objective is to minimize the total cost,