MPI-I-1999-1-005

Two improved list-ranking algorithms are presented. The ``peeling-off'' algorithm leads to an optimal PRAM algorithm, but was designed with application on a real parallel machine in mind. It is simpler than earlier algorithms, and in a range of problem sizes, where previously several algorithms where required for the best performance, now this single algorithm suffices. If the problem size is much larger than the number of available processors, then the ``sparse-ruling-sets'' algorithm is even better. In previous versions this algorithm had very restricted practical application because of the large number of communication rounds it was performing. This main weakness of this algorithm is overcome by adding two new ideas, each of which reduces the number of communication rounds by a factor of two.