1. February 2002Parallel GRASP for the 2-path network design problem Slide 1/25 (ROADEF) 4 èmes Journées de la ROADEF Paris, February 20-22, 2002 A Parallel GRASP Heuristic for the 2-Path Network Design Problem Celso C. RIBEIRO Isabel ROSSETI Catholic University of Rio de Janeiro Brazil Metro Corvisart, Paris 13 ème

2. February 2002Parallel GRASP for the 2-path network design problem Slide 2/25 (ROADEF) Summary Problem formulation GRASP with path-relinking heuristic –Construction phase –Local search phase –Path-relinking Parallel implementation Computational results Concluding remarks

3. February 2002Parallel GRASP for the 2-path network design problem Slide 3/25 (ROADEF) 2-path network design problem Graph G = (V,E) V: node set E: edge set weights w e associated with each edge e  E k-path between nodes s,t  V: sequence of at most k edges connecting s and t D: set of demands (origin-destination pairs)

4. February 2002Parallel GRASP for the 2-path network design problem Slide 4/25 (ROADEF) 2-path network design problem 2-path network design problem (2PNDP): Find a minimum weighted subset of edges E’  E containing a 2-path in G between the extremities of every origin-destination pair in D Applications: design of communication networks, in which paths with few edges are sought to enforce high reliability and small delays

5. February 2002Parallel GRASP for the 2-path network design problem Slide 5/25 (ROADEF) 2-path network design problem Dahl & Johannessen (2000): –Decision version of 2PNDP is NP- complete. –Approximate algorithm –Exact cutting plane algorithm Balakrishnan & Altinkemer (1992): –Integer programming formulation for kPNDP –See also LeBlanc, Chifflet & Mahey (1999). Generalizations: k-hop minimum spanning tree, k-hop minimum Steiner tree

6. February 2002Parallel GRASP for the 2-path network design problem Slide 6/25 (ROADEF) GRASP with path-relinking GRASP: –Multistart metaheuristic: Feo & Resende (1989) Path-relinking: –Intensification strategy: Glover (1996) Repeat for Max_Iterations: –Construct a greedy randomized solution –Use local search to improve the constructed solution –Apply path-relinking to further improve the solution –Update the pool of elite solutions –Update the best solution found

7. February 2002Parallel GRASP for the 2-path network design problem Slide 7/25 (ROADEF) GRASP with path-relinking GRASP –Construction phase 1.Set the modified weights equal to the original weights. 2.Randomly select an origin-destination pair (a,b)  D. 3.Compute a shortest 2-path between a and b using the modified weights. 4.Set to 0 the modified weights of the edges in this path. 5.Remove (a,b) from D. 6.If D is empty stop, otherwise go back to step 2.

8. February 2002Parallel GRASP for the 2-path network design problem Slide 8/25 (ROADEF) GRASP with path-relinking GRASP –Local search phase 1.Generate a circular random permutation of the pairs in D. 2.Select the next origin-destination pair (a,b)  D. 3.Tentatively replace the shortest 2-path between a and b: Weights of edges used by other 2-paths are temporarilly set to 0. Compute a new shortest 2-path between a and b. Update the current solution if it is improved by the new 2-path. Restore all original edge weights. 4.If |D| paths have been investigated without improvement stop, otherwise go back to step 2.

9. February 2002Parallel GRASP for the 2-path network design problem Slide 9/25 (ROADEF) GRASP with path-relinking Path-relinking: introduced in the context of tabu search by Glover (1996) –Intensification strategy using set of elite solutions Consists in exploring trajectories that connect high quality solutions. initial solution guiding solution path in neighborhood of solutions

10. February 2002Parallel GRASP for the 2-path network design problem Slide 10/25 (ROADEF) GRASP with path-relinking Path is generated by selecting moves that introduce in the initial solution attributes of the guiding solution. At each step, all moves that incorporate attributes of the guiding solution are evaluated and the best move is taken: Initial solution guiding solution

11. February 2002Parallel GRASP for the 2-path network design problem Slide 11/25 (ROADEF) Elite solutions x and y  (x,y): symmetric difference between x and y while ( |  (x,y)| > 0 ) { evaluate moves corresponding in  (x,y) make best move update  (x,y) } GRASP with path-relinking

12. February 2002Parallel GRASP for the 2-path network design problem Slide 12/25 (ROADEF) GRASP with path-relinking Maintain an elite set of solutions found during GRASP iterations. After each GRASP iteration (construction and local search): –Select an elite solution at random: guiding solution. –Use GRASP solution as initial solution. –Perform path-relinking between these two solutions.

13. February 2002Parallel GRASP for the 2-path network design problem Slide 13/25 (ROADEF) GRASP with path-relinking Successful applications: –Prize-collecting Steiner tree problem: Canuto, Resende & Ribeiro (2001) –Minimum Steiner tree problem: Ribeiro, Uchoa & Werneck (2002) (e.g., best known results for open problems in series dv640 of the SteinLib) –Three-index assignment problem: Aiex et al. (2000) –Capacitated minimum spanning tree: Souza, Duhamel & Ribeiro (2002) (e.g., best known results for largest problems with 160 nodes)

14. February 2002Parallel GRASP for the 2-path network design problem Slide 14/25 (ROADEF) GRASP with path-relinking P is a set of elite solutions. Each iteration of first |P| GRASP iterations adds one solution to P (if different from others). After that: solution x is promoted to P if: –x is better than best solution in P. –x is not better than best solution in P, but is better than worst and is sufficiently different from all solutions in P.

15. February 2002Parallel GRASP for the 2-path network design problem Slide 15/25 (ROADEF)

16. February 2002Parallel GRASP for the 2-path network design problem Slide 16/25 (ROADEF) Parallel implementation Main interest of parallel implementations of metaheuristics: robustness Cung, Martins, Ribeiro & Roucairol (2001) Parallelization strategy: –Multiple-walk independent-thread strategy –Iterations evenly distributed over p processors –Each processor keeps a copy of the algorithm and data –One processor acts as the master (data, seeds, iterations) –Each processor performs Max_Iterations/p iterations

17. February 2002Parallel GRASP for the 2-path network design problem Slide 17/25 (ROADEF) Computational results Parallel GRASP heuristic: –Implementation in C –MPI LAM 6.3.2 for communication –Linux cluster with 32 Pentium II-400 processors Largest instances solved: –Larger instances solved with the GRASP heuristic: |V|= 400, |E|= 79800, |D|= 4000 (previously: |V|= 120, |E|= 7140, |D|= 60)

18. February 2002Parallel GRASP for the 2-path network design problem Slide 18/25 (ROADEF) Computational results Effectiveness: –100 small instances with 70 nodes generated as in Dahl and Johannessen (2000) for comparison purposes. –Statistical test t for unpaired observations –Parallel GRASP finds better solutions with 40% of confidence. Parall el GRAS P Sample A D&J (2000 ) Sample B Size10030 Mean443.7 (- 2.2%) 453.7 Std. dev. 40.661.6

19. February 2002Parallel GRASP for the 2-path network design problem Slide 19/25 (ROADEF) Variants of GRASP with path- relinking: –GRASP: pure GRASP –G+PR(B): GRASP with backward PR –G+PR(F): GRASP with forward PR –G+PR(BF): GRASP with two-way PR Other strategies: –Truncated path-relinking –Do not apply PR at every iteration (frequency) Variants of GRASP with path- relinking S T T S S T S T

20. February 2002Parallel GRASP for the 2-path network design problem Slide 20/25 (ROADEF) Variants of GRASP with path- relinking Select an instance and a target value. For each variant of GRASP with path- relinking: –Perform 200 runs using different seeds. –Stop when a solution value at least as good as the target is found. –For each run, measure the time-to- target-value. –Plot the probabilities of finding a solution at least as good as the target value within some computation time.

21. February 2002Parallel GRASP for the 2-path network design problem Slide 21/25 (ROADEF) Variants of GRASP with path- relinking Each variant: 200 runs for one instance of 2PNDP

22. February 2002Parallel GRASP for the 2-path network design problem Slide 22/25 (ROADEF) Variants of GRASP with path- relinking Same computation time: probability of finding a solution at least as good as the target value increases from GRASP  G+PR(F)  G+PR(B)  G+PR(BF) P(h,t) = probability that variant h finds a solution as good as the target value in time no greater than t –P(GRASP,10s) = 2% P(G+PR(F),10s) = 56% P(G+PR(B),10s) = 75% P(G+PR(BF),10s) = 84% Effectiveness of path-relinking to improve and speedup the pure GRASP

23. February 2002Parallel GRASP for the 2-path network design problem Slide 23/25 (ROADEF) Speedups Linear speedups: |V|= 400, 3200 iterations

24. February 2002Parallel GRASP for the 2-path network design problem Slide 24/25 (ROADEF) Concluding remarks New heuristic for the 2-path network design problem. Effectiveness of the new heuristic: –Larger problems solved. –New heuristic finds better solutions. –Domination is stronger for harder or larger instances. Path-relinking adds memory and intensification mechanisms to GRASP, systematically contributing to improve solution quality (some implementation strategies appear to be more effective than others). Linear speedups with the parallel implementation.

25. February 2002Parallel GRASP for the 2-path network design problem Slide 25/25 (ROADEF) Slides and publications Slides of this talk can be downloaded from: http://www.inf.puc- rio/~celso/talks Paper about the parallel GRASP heuristic for the 2-path network design problem available at: http://www.inf.puc- rio.br/~celso/publicacoes (after March 15, 2002) Isabel Rosseti