ILOG CPLEX 11.0 User's Manual

directives about the order in which to branch on variables. In a priority order, you can alsoprovide preferred branching directions for specific variables.Solving SubproblemsILOG CPLEX allows you to distinguish the algorithm applied to the initial relaxation ofyour problem from the algorithm applied to other continuous subproblems of a MIP. Thisdistinction between initial relaxation and the other MIP subproblems may be useful whenyou have special information about the nature of your model. In this context, "other MIPsubproblems" includes nodes of the branch & cut tree, problems re-solved after cuttingplane passes, problems solved by node heuristics, and so forth.The parameter RootAlg (CPX_PARAM_STARTALG) enables you to specify which algorithmfor ILOG CPLEX to apply to the initial relaxation.The parameter NodeAlg (CPX_PARAM_SUBALG) lets you specify the algorithm applied toother continuous subproblems.For more detail about these parameters, see Unsatisfactory Optimization of Subproblems onpage 297.Using Node FilesOn difficult models that generate a great number of nodes in the tree, the amount of availablememory for node storage can become a limiting factor. Node files can be an effectivetechnique which uses disk space to augment RAM, at little or no penalty in terms of solutionspeed.The node-file storage-feature enables you to store some parts of the branch & cut tree in fileswhile the branch & cut algorithm is being applied. If you use this feature, ILOG CPLEXwill be able to explore more nodes within a smaller amount of computer memory. Thisfeature includes several options to reduce the use of physical memory, and it entails a verysmall increase in runtime. Node-file storage as managed by ILOG CPLEX itself offers amuch better option in terms of memory use and performance time than relying on swapspace as managed by your operating system in this context.For more about the parameters controlling node files, see Use Node Files for Storage onpage 293.ProbingThe probing feature can help in many different ways on difficult models. Probing is atechnique that looks at the logical implications of fixing each binary variable to 0 (zero) or1 (one). It is performed after preprocessing and before the solution of the root relaxation.Probing can be expensive, so this parameter should be used selectively. On models that arein some sense easy, the extra time spent probing may not reduce the overall time enough tobe worthwhile. On difficult models, probing may incur very large runtime costs at the270 ILOG CPLEX 11.0 — USER’ S MANUAL