Speeding Up the NSGA-II with a Simple Tie-Breaking Rule

Abstract

The non-dominated sorting genetic algorithm II (NSGA-II) is the most popular multi-objective optimization heuristic. Recent mathematical runtime analyses have detected two shortcomings in discrete search spaces, namely, that the NSGA-II has difficulties with more than two objectives and that it is very sensitive to the choice of the population size. To overcome these difficulties, we analyze a simple tie-breaking rule in the selection of the next population. Similar rules have been proposed before, but have found only little acceptance. We prove the effectiveness of our tie-breaking rule via mathematical runtime analyses on the classic ONEMINMAX, LEADINGONESTRAILINGZEROS, and ONEJUMPZEROJUMP benchmarks. We prove that this modified NSGA-II can optimize the three benchmarks efficiently also for many objectives, in contrast to the exponential lower runtime bound previously shown for ONEMINMAX with three or more objectives. For the bi-objective problems, we show runtime guarantees that do not increase when moderately increasing the population size over the minimum admissible size. For example, for the ONEJUMPZEROJUMP problem with representation length n and gap parameter k, we show a runtime guarantee of O(max{nk+1, Nn}) function evaluations when the population size is at least four times the size of the Pareto front. For population sizes larger than the minimal choice N = Θ(n), this result improves considerably over the Θ(Nnk) runtime of the classic NSGA-II.