ILOG CPLEX 11.0 User's Manual

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parameter settings are also worth considering for this symptom of difficulty finding morethan one feasible solution.Large Number of Unhelpful CutsWhile the cuts added by ILOG CPLEX reduce runtime for most problems, on occasion theycan have the opposite effect. If you notice, for example, that ILOG CPLEX adds a largenumber of cuts at the root, but the objective value does not change significantly, then youmay want to experiment with turning off cuts selectively (that is, by type of cut) orcompletely.To turn off cuts selectively, use the cut parameters summarized in Table 14.9 on page 274,with a value of -1 (minus one). For example, in the Interactive Optimizer, the followingcommand turns off only cover cuts:set mip cuts covers -1To limit types of cuts generated, consider the parameter EachCutLim,CPX_PARAM_EACHCUTLIM.To limit the number of passes that ILOG CPLEX executes to generate cuts, consider theparameter CutPass, CPX_PARAM_CUTPASS.To turn off all cuts, set the cut pass parameter to -1 (minus one). For example, in theInteractive Optimizer, use the following command to turn off all generation of all cuts:set mip cuts all -1Lack of Movement in the Best NodeFor some models, the Best Node value in the node log changes very slowly or not at all.Runtimes for such models can sometimes be reduced by the variable selection strategyknown as strong branching. Strong branching explores a set of candidatebranching-variables in-depth, performing a limited number of simplex iterations to estimatethe effect of branching up or down on each.Important: Strong branching consumes significantly more computation time per node thanthe default variable selection strategy.To activate strong branching, set the VarSel parameter to a value of 3.On rare occasions, it can be helpful to modify strong branching limits. If you modify thelimit on the size of the candidate list, then strong branching will explore a larger (or smaller)set of candidates. If you modify the limit on strong branching iteration, then strongbranching will perform more (or fewer) simplex iterations per candidate.290 ILOG CPLEX 11.0 — USER’ S MANUAL
These limits are controlled by the parameters StrongCandLim and StrongItLim,respectively.Other parameters to consider trying, in the case of slow movement of the Best Node value,are non-default levels for Probe (try the aggressive setting of 3 first, and then reduce it if theprobing step itself takes excessive time for your purposes), and MIPEmphasis set to a valueof 3.Time Wasted on Overly Tight Optimality CriteriaSometimes ILOG CPLEX finds a good integer solution early, but must examine manyadditional nodes to prove the solution is optimal. You can speed up the process in such acase if you are willing to change the optimality tolerance. ILOG CPLEX supports two kindsof tolerance:◆Relative optimality tolerance guarantees that a solution lies within a certain percentageof the optimal solution.◆ Absolute optimality tolerance guarantees that a solution lies within a certain absoluterange of the optimal solution.The default relative optimality tolerance is 0.0001. At this tolerance, the final integersolution is guaranteed to be within 0.01% of the optimal value. Of course, manyformulations of integer or mixed integer programs do not require such tight tolerance, sorequiring ILOG CPLEX to seek integer solutions that meet this tolerance in those cases iswasted computation. If you can accept greater optimality tolerance in your model, then youshould change the parameter EpGap.If, however, you know that the objective values of your problem are near zero, then youshould change the absolute gap because percentages of very small numbers are less useful asoptimality tolerance. Change the parameter EpAGap in this case.To speed up the proof of optimality, you can set objective difference parameters, bothrelative and absolute. Setting these parameters helps when there are many integer solutionswith similar objective values. For example, setting the ObjDif parameter to 100.0 makesILOG CPLEX skip any potential solution with its objective value within 100.0 units of thebest integer solution so far. Or, setting the RelObjDif to 0.01 would mean thatILOG CPLEX would skip any potential new solution that is not at least 1% better than theincumbent solution. Naturally, since this objective difference setting may makeILOG CPLEX skip an interval where the true integer optimum may be found, the objectivedifference setting weakens the guarantee of optimality.Cutoff parameters can also be helpful in restricting the search for optimality. If you knowthat there are solutions within a certain distance of the initial relaxation of your problem,then you can readily set the upper cutoff parameter for minimization problems and the lowercutoff parameter for maximization problems. Set the parameters CutUp and CutLo,respectively, to establish a cutoff value.ILOG CPLEX 11.0 — USER’ S MANUAL 291
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ILOG CPLEX 11.0User’s ManualSepte
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C O N T E N T STable of ContentsILO
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Building the Model by Column . . .
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Program Description . . . . . . . .
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Cholesky Factor in the Log File . .
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Examples: QCP . . . . . . . . . . .
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Chapter 18 Using Piecewise Linear F
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Representing the Data. . . . . . .
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Reading and Writing MPS Files . . .
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Protected Variables in Presolve Red
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P R E F A C EMeet ILOG CPLEXILOG CP
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When a linear optimization problem
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◆◆◆◆implemented in ILOG CPL
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Solution Pool: Generating and Keepi
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Table 1ExamplesExample Source File
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◆◆◆◆◆◆Overview of the A
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Wolsey, Laurence A., Integer Progra
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C H A P T E R1ILOG Concert Technolo
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Compiling and LinkingCompilation an
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The class IloNumVar provides method
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IloModel object itself and added to
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Table 1.1 Concert Technology Modeli
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Choosing an OptimizerSolving the ex
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Table 1.4 on page 55 summarizes the
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Dynamic control of the solution pro
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However, querying solution values v
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parameter for the getQuality method
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model2.add(model1);model2.add(IloCo
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Example: Optimizing the Diet Proble
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Concert Technology, as demonstrated
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method IloModel::add returns the mo
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Licenses in a Java ApplicationILOG
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Models that consist only of such co
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The special case of linear expressi
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IloObjective obj = cplex.add(cplex.
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Solving the ModelOnce you have crea
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IloCplex cplex = new IloCplex();Ilo
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IloCplex.setParam(IloCplex.IntParam
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Thus, the suggested method for sett
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Basis InformationWhen solving an LP
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where A is a sparse matrix. A spars
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The example starts by evaluating th
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IloMPModeler.setLinearCoefs, and Il
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◆What is the purpose (the objecti
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Step 3Create the modelGo to the com
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Step 9Add nutritional constraintsGo
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Step 14Display the solutionGo to th
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Step 18Enclose the application in t
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C H A P T E R4ILOG CPLEX Callable L
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◆◆file reading and writing rout
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◆◆If data already exist in MPS,
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For example, let’s look at the sy
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As you modify a problem object thro
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However, ILOG CPLEX updates the nam
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whether an optimization should be a
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Callable Library have names greater
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◆CPXsetstrparam accepts arguments
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Here’s another way to visualize a
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program—can discover the length o
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Part IIProgramming ConsiderationsTh
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◆ Test Data on page 135◆ Choose
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◆ Use the MIP optimizer if the pr
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Choose Clarity First, Efficiency La
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1. If you can reproduce the behavio
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indices, errors will occur. Therefo
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Understanding File FormatsThe refer
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definitions that it finds. The firs
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◆◆IloXmlReader creates a reader
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Controlling Message ChannelsIn both
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more calls to the message handling
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Concert Technology Message Channels
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Types of ILM Runtime LicensesILM ru
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char *inststr = NULL;char *envstr =
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The registerLicense Method for Java
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not correct that problem either. In
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In the Interactive Optimizer, you c
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For models not in the current worki
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◆◆In the .NET API, pass an inst
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C H A P T E R9Solving LPs: Simplex
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The symbolic names for these settin
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When no candidates are present, the
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solution; but examination of soluti
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Then to later read an advanced basi
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Table 9.5 PPriInd Parameter Setting
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ILOG CPLEX ignores the coefficients
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Numeric DifficultiesILOG CPLEX is d
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esult is the same as would be obtai
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automatically perturbs the variable
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4. Consider alternate scalings.You
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smaller in absolute value than the
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exceptions are caught as well, and
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C H A P T E R10Solving LPs: Barrier
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The ILOG CPLEX Barrier Optimizer is
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And then you call the solution rout
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Here is an example of a log file fo
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If solution values are large in abs
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Normalized errors, for example, rep
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◆ workmem in the Interactive Opti
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est order. It may require more time
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choice of starting-point heuristic
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To see the current value of the col
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C H A P T E R11Solving Network-Flow
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◆◆l a , the lower bound, sets t
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of the objective function calculate
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then ILOG CPLEX will carry out such
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-a 1 + a 2 - a 8 - a 9 + a 14 = 0-
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C H A P T E R12Solving Problems wit
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convex QPs, Q must be positive semi
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A similar Java program using Concer
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◆ qp indicates that you want ILOG
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RootAlg parameter (QPMETHOD in the
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ilolpex1.cpp counterpart. Here the
Page 239 and 240: C H A P T E R13Solving Problems wit
Page 241 and 242: Figure 13.2y(0, 1)(-1, 0)d(1, 0)xc(
Page 243 and 244: Detecting Problem TypeILOG CPLEX de
Page 245 and 246: COLUMNSMARK0000 'MARKER''INTORG'C15
Page 247 and 248: C180 R100 159C180 R119 200C180 R120
Page 249 and 250: QMATRIXC158 C158 1C158 C189 0.5C189
Page 251 and 252: ◆From the Callable Library, use t
Page 253: Part IVDiscrete OptimizationThis pa
Page 256 and 257: Stating a MIP ProblemA mixed intege
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Page 260 and 261: ◆milp, miqp, or miqcpindicating t
Page 262 and 263: 1. finding a succession of improvin
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Page 266 and 267: If the solution to the relaxation s
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Page 270 and 271: directives about the order in which
Page 272 and 273: ◆ Parameters Affecting Cuts on pa
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Page 276 and 277: Relaxation Induced Neighborhood Sea
Page 278 and 279: Table 14.10Parameters for Controlli
Page 280 and 281: Library). If this process succeeds,
Page 282 and 283: After you have solved a MIP, you wi
Page 284 and 285: Table 14.13Settings of the MIP Disp
Page 286 and 287: ILOG CPLEX also logs its addition o
Page 288 and 289: ◆ Parent specifies the NodeID of
Page 292 and 293: When you set a MIP cutoff value, IL
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Page 296 and 297: ◆◆●●at problem modification
Page 298 and 299: ◆In the Callable Library, use the
Page 300 and 301: ●●Then call CPXprimopt to optim
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Page 304 and 305: c17: x1 - y11 >= 0c18: x1 - y21 >=
Page 306 and 307: Populating the Solution PoolILOG CP
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Page 310 and 311: Note: The parameter to limit the nu
Page 312 and 313: Example: Using Populate after MIP O
Page 314 and 315: Limitations Due to Numeric Difficul
Page 316 and 317: Examining the Solution PoolIn the I
Page 318 and 319: For a sample of these methods or ro
Page 320 and 321: ◆◆In the Callable Library (C AP
Page 322 and 323: Then display the objective value of
Page 324 and 325: ◆ If you want to filter solutions
Page 326 and 327: ●●●In the C++ API, use the me
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Page 330 and 331: continuous, a model containing one
Page 332 and 333: ◆The routine setPriorities sets t
Page 334 and 335: What Are Semi-Continuous Variables?
Page 336 and 337: With that model, now the applicatio
Page 338 and 339: Piecewise Linearity in ILOG CPLEXSo
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Reminder: It may help you understan
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overlaps with an endpoint of two ot
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Figure 18.4400003000020000100000Fig
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in demand; in terms of this model,
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348 ILOG CPLEX 11.0 — USER’ S M
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What Are Logical Constraints?For IL
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◆◆Cplex.NotCplex.IfThenAgain, t
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In Concert Technology applications,
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Describing the ProblemThe problem i
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Notice that how much to use and buy
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Then use a for-loop to add the cons
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These lines (the action of the if-s
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Describing the ProblemThe problem i
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Stating Precedence ConstraintsIn ea
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Solving the ProblemAn emphasis on f
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What Is Column Generation?In colloq
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Solving this model with all columns
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Likewise, the application adds an a
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Pattern Generator ModelThe submodel
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Complete ProgramYou can see the ent
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your problem formulation caused thi
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To control the types of reductions
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What Is Unboundedness?Any class of
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Callable Library use the advanced r
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the conflict to arrive at a minimal
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the IIS finder can, and often more.
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Then you will see results like thes
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Now view the entire conflict with t
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The constraints in conflict with th
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If a model contains more than one c
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Immediately after that statement, i
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◆ Groups in the Conflict Refiner
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The infeasibility on which FeasOpt
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Now the following lines invoke Feas
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suggests decreasing the upper bound
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C H A P T E R28User-Cut and Lazy-Co
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However, there is an important dist
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◆◆Through the routine CPXreadco
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Here is an example of an MPS file e
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C H A P T E R29Using GoalsThis chap
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How Goals Are Implemented in Branch
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In the Java API, a Fail goal is ret
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This return statement returns an An
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The Goal StackTo understand how goa
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soon as they are enclosed in a goal
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The global cut goal for lhs[i] ≤
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If a node has multiple evaluators a
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As this example is an extension of
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C H A P T E R30Using Optimization C
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Reference Documents about Informati
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Query or Diagnostic CallbacksQuery
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● Cplex.DisjunctiveCutCallback in
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◆◆●●Cplex.HeuristicCallback
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It is not customary to write such a
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IloCplex::Callback mycallback = cpl
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Here is the previous sample of code
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◆◆cbdata, a pointer to ILOG CPL
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conventions of the Interactive Opti
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een added, it can be deleted by cal
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Interaction Between Callbacks and I
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C H A P T E R31Goals and Callbacks:
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The only functionality that is not
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C H A P T E R32Advanced Presolve Ro
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once. All of the node relaxation so
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- 5x1, + x2 ≤ 0 2y1, + y2, ≥ 1x
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problem, the problem has a presolve
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Modifying a ProblemThis section bri
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C H A P T E R33Advanced MIP Control
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value strictly greater than one. It
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Cut CallbackThe next example we con
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problem. Since branching involves a
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C H A P T E R34Parallel OptimizersT
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Threads and Performance Considerati
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3. Call the parallel optimizer with
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optimizer turns out to be the faste
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0 0 3383.7784 16 4505.0000 Cuts: 35
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I N D E XIndexAabsolute objective d
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ole in converting LP to network flo
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emoving from basis (C++ API) 62repr
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CPX_PARAM_PCPX_PARAM_POLISHTIMEsolu
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CPXcreateprob 467CPXcreateprob rout
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CPXwcpxwarning message channel 151C
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empty goal 431, 435end methodIloEnv
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getDuals method (Java API) 88getMax
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emove method (Java API) 94IloModele
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ecords singularities 188relocating
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from 218head 218sink 219source 219s
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control callbacks and 453query call
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primal reduction 385primal simplex
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eturn statusBounded (Java API) 81Er
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solvingdiet problem (Java API) 82mo
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arrier 208WorkMem 293WorkMem parame
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