We study efficient implementations of the push-relabel method for the
maximum flow problem. The resulting codes are faster than the previous
codes, and much faster on some problem families. The speedup is due t
o the combination of heuristics used in our implementations: we show t
hat the highest-level selection strategy gives better results when com
bined with both global and gap relabeling heuristics. We also exhibit
a family of problems for which the running time of all implementations
we consider is quadratic.