Abstract SPEA-II (Strength Pareto Evolutionary Algorithm 2) is an improved multi-objective evolutionary algorithm that differs from NSGA-II in its selection mechanism. While NSGA-II uses non-dominated sorting and crowding distance, SPEA-II maintains an external archive to preserve elite non-dominated solutions and uses a fine-grained fitness assignment strategy based on the strength of domination.
Note that when using warm-start with existing trials, the initial generation may not support concurrent sampling. After the initial generation, the implementation follows standard evolutionary algorithm parallelization.