The authors would like to thank the anonymous reviewers for their helpful comments to improve the quality of the paper. This research was performed in part using the Intel iPSC/860 hypercube and the Paragon computers at the Oak Ridge National Laboratory, and in part using the Intel Touchstone Delta system operated by the California Institute of Technology on behalf of the Concurrent Supercomputing Consortium. Access to the Delta system was provided through the Center for Research on Parallel Computing.