ILOG CPLEX 11.0 User's Manual

anches taken so far in this dive. Setting BtTol to a value near 0.0 increases the likelihoodthat a backtrack will occur, while the default value near 1.0 makes it more likely that thepresent dive will continue to a resolution (fathoming either via a cutoff or an infeasiblecombination of branches or the discovery of a new incumbent integer feasible solution). Seethe reference manual ILOG CPLEX Parameters for more details about how this parameterinfluences the computation that makes the decision to backtrack.Selecting NodesWhen ILOG CPLEX backtracks, there usually remain large numbers of unexplored nodesfrom which to begin a new dive. The node selection parameter, NodeSel, sets this choice.Table 14.6NodeSel Parameter Settings for Node Search TypeNodeSelValueSymbolic ValueNode Search Type1 (Default) CPX_NODESEL_BESTBOUND Best Bound search, which means thatthe node with the best objective functionwill be selected, generally near the topof the tree.2 CPX_NODESEL_BESTEST Best Estimate search, wherebyILOG CPLEX will use an estimate of agiven node's progress toward integerfeasibility relative to its degradation ofthe objective function. This setting canbe useful in cases where there isdifficulty in finding feasible solutions orin cases where a proof of optimality isnot crucial.3 CPX_NODESEL_BESTEST_ALT A variation on the Best Estimate search.0 CPX_NODESEL_DFS Depth First search will be conducted. Inmany cases this amounts to a bruteforce strategy for solving thecombinatorial problem, gaining a smallamount of tactical efficiency due to avariety of reasons, and it is rare that itoffers any advantage over othersettings.In instances where Best Estimate node selection (NodeSel = 2 or 3) is in effect, theBBInterval parameter sets the frequency at which backtracking is done by Best Boundanyway. The default value of 7 works well, but you can set it to 0 (zero) to make sure thatBest Estimate is used every time backtracking occurs.268 ILOG CPLEX 11.0 — USER’ S MANUAL