Shortest Paths and Multicommodity Network Flows

Chapter 6 Primal-dual column generation and partitioning methods

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        We have briefly introduced the primal-dual methods in Section 2.4 and key variable decomposition in Section 2.3.3. In this chapter, we will combine these two methods to solve MCNF problems.


        In this chapter we first discussed generic primal-dual methods (PD) for solving origin-destination multicommodity network flow (ODMCNF) problems. We proposed two methods to determine the step length theta^{*} and choose the latter one for our implementation. We perturbed the objective coefficients of the artificial variables to resolve the degenerate pivots caused by primal degeneracy.

        We then proposed a new method, the primal-dual key path method (KEY), to solve ODMCNF problems. We discussed its properties and difficulties, and proposed techniques to resolve cycling problems caused by dual degeneracy.
We compared our algorithm KEY with other three algorithms, PD, DW, and NA. We found DW, the Dantzig-Wolfe decomposition method, was the fastest algorithm in all of our tests. As expected, solving the node-arc formulation (NA) without any special techniques such as resource-directive methods or the basis-partitioning methods introduced in Chapter 2, will be time-consuming and very inefficient, except for small networks.

        KEY follows the generic PD steps but uses the key path decomposition method to solve a relaxed RPP at each iteration. It is designed for problems with large number of commodities. In our limited tests, KEY does perform better than PD for cases with more commodities. However, for larger cases with few commodities, it performs much worse than PD. The reason for its inefficiency is that it requires many iterations of key path swapping operations. Although our proposed technique, which perturbs the objective coefficients for path flows, has reduced the chances of key path swapping and resolved the cycling problem caused by dual degeneracy, it is still not efficient enough for some larger cases.
More tests should be performed, especially using cases with a large number of commodities, to draw more solid conclusions on the efficiency of KEY.

        We also found that generating all shortest paths for the same commodity may not save time in Dantzig-Wolfe decomposition with a column generation scheme. That is, generating a single shortest path for each commodity is good enough.

        To explain why DW performs so well compared to PD and KEY, we give the following reasons:

First, DW generates fewer shortest paths than DW and KEY. In particular, DW will not generate a path twice, if we keep all of the previously generated paths in the restricted master problem. PD and KEY, on the other hand, generate a new set of all shortest paths at every iteration using the new dual solutions. Note that between two primal-dual iterations, many paths may remain the shortest and do not need to be altered. The reason that we remove the entire previous set of shortest paths and add a new set of shortest paths is that keeping old paths would require bookkeeping on each specific path, which is difficult and inefficient. In particular, to check whether a path is in the current set takes exponential time when there are (exponentially) many paths to be added or removed. Furthermore, using CPLEX to solve the RPP usually requires the sparse representation of the constraint matrix. In this case, each path corresponds to a column. Removing and adding columns will require a new sparse representation. Although we can add easily columns at the end of the matrix, identifying the columns to be removed requires sparse matrix operations. Thus, PD and KEY invoke many more operations than DW does, making the algorithms less efficient.

        Degeneracy is an important issue, especially for primal-dual based methods like PD and KEY. Multiple optimal dual or primal solutions may cause implementation problems. Techniques that choose a "good" optimal solution are desired, and may drastically shorten the total running time.

        KEY has one more issue of concern than PD: the memory requirement. To implement the key path swapping operations more efficiently, we allocate memory to store the initial relaxed RPP at each primal-dual iteration. Inside a primal-dual iteration, KEY may perform many iterations of key path swapping. The column corresponding to the key path is empty, and an empty column has no sparse representation at all. Thus, it is clumsy to do the key path swapping and column operations only using the original sparse representation of the RPP. Although we have an efficient way to swap key paths and update each column for the commodities that have new key paths, it still requires much time, especially when there are many iterations of key path swapping operations. Table 3.3 and Table 3.4 show that more iterations of swapping key paths results in a longer running time compared to PD.

        To make KEY more competitive, new techniques that identify better dual improving directions can be developed. Our results show that efficient MPSP algorithms play a crucial role in improving all of the path-oriented multicommodity network flow algorithms, especially primal-dual based algorithms like PD and KEY which 85% (PD) or 95% (KEY) of their total running time computing shortest paths.