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Contrasting Structured and Random Permutation Flow-Shop Scheduling Problems: Search-Space Topology and Algorithm Performance

Department of Computer Science, Colorado State University, Fort Collins, Colorado 80523, USA
Department of Computer Science, Colorado State University, Fort Collins, Colorado 80523, USA
Department of Computer Science, Colorado State University, Fort Collins, Colorado 80523, USA
Department of Computer Science, Colorado State University, Fort Collins, Colorado 80523, USA
watsonj{at}cs.colostate.edu
laura{at}cs.colostate.edu
whitley{at}cs.colostate.edu
howe{at}cs.colostate.edu

The use of random test problems to evaluate algorithm performance raises an important, and generally unanswered, question: Are the results generalizable to more realistic problems? Researchers generally assume that algorithms with superior performance on difficult, random test problems will also perform well on more realistic, structured problems. Our research explores this assumption for the permutation flow-shop scheduling problem. We introduce a method for generating structured flow-shop problems, which are modeled after features found in some real-world manufacturing environments. We perform experiments that indicate significant differences exist between the search-space topologies of random and structured flow-shop problems, and demonstrate that these differencescanaffect the performance of certain algorithms. Yet despite these differences, and in contrast to difficult random problems, the majority of structured flow-shop problems were easily solved to optimality by most algorithms. For the problems not optimally solved, differences in performance were minor. We conclude that more realistic, structured permutation flow-shop problems are actually relatively easy to solve. Our results also raise doubts as to whether superior performance on difficult random scheduling problems translates into superior performance on more realistic kinds of scheduling problems.

Key words: Flow Shop; Sequencing; Local Search; Problem Difficulty; Fitness Landscapes
History: received November 2000; revised June 2001; accepted October 2001.