Semi-clairvoyant Scheduling

In [2] it was shown that the obvious semi-clairvoyant generalization of the Shortest Processing Time is O(1)-competitive with respect to average stretch on a single machine. In [2] it was left as an open question whether it was possible for a semiclairvoyant algorithm to be O(1)-competitive with respect to average flow time on a single machine. Here we settle this open question by giving a semi-clairvoyant algorithm that is O(1)-competitive with respect to average flow time on a single machine. We also show a semi-clairvoyant algorithm on parallel machines that achieves up to constant factors the best known competitive ratio for clairvoyant on-line algorithms. In some sense one might conclude from this that the QoS achievable by semi-clairvoyant algorithms is competitive with clairvoyant algorithms. We finally show that, in contrast to the clairvoyant case, no semi-clairvoyant algorithm can be simultaneously O(1)-competitive with respect to average stretch and O(1)competitive with respect to average flow time.