Abstract. We study online preemptive makespan minimization on m parallel machines, where the (multiprocessor) jobs arrive over time and have widths from some fixed set W ⊆ {1, 2, . . . , m}. For every number m of machines we concisely characterize all the sets W for which there is a 1-competitive fully online algorithm and all the sets W for which there is a 1-competitive nearly online algorithm.