Issues with using IloCP with too many workers

Question

I have built a simple constraint enforcing that the value of a variable must be consistent with a modulo operation. Two consistency levels are available: bounds consistency and domain consistency. In this example, solving the problem with bounds consistency works well with 1 or 2 workers, but with 3 or more workers the propagate() function loops infinitely since it appears to be unable to remove the maximum value of the domain. My computer has 2 physical cores, so my guess is that using more workers than physical cores is the problem. I am using the latest version of Cplex Optimization Studio. Here is the code:

File main.cc:

#include <ilcp/cp.h>
#include "IloModulo.h"


int main() {
  IloEnv env;
  IloModel model(env);

  IloIntVar x(env, 1, 100000);
  model.add(IloModulo(env, x, 3, BOUNDS_CONSISTENCY));
  model.add(IloMinimize(env, x));

  IloCP solver(model);
  solver.setParameter(IloCP::Workers, 1); // Infinite loop with 3 or more workers
  solver.solve();

  return 0;
}

File IloModulo.h:

#ifndef ILOMODULO_H_
#define ILOMODULO_H_


#include <ilcp/cpext.h>


#define BOUNDS_CONSISTENCY 1
#define DOMAIN_CONSISTENCY 2


class IlcModuloI : public IlcConstraintI {
private:
  IlcIntVar x_;
  IlcInt modulo_;
  IlcInt consistency_;

public:
  IlcModuloI(IloCPEngine cp,
         IlcIntVar x,
         IlcInt modulo,
         IlcInt consistency);
  ~IlcModuloI();

  virtual void propagate();
  virtual void post();
};

// The IloModulo constraint is used with the following function
IloConstraint IloModulo(IloEnv env,
            IloIntVar x,
            IloInt modulo,
            IloInt consistency);


#endif // ILOMODULO_H_

File IloModulo.cc:

#include "IloModulo.h"


IlcModuloI::IlcModuloI(IloCPEngine solver,
               IlcIntVar x,
               IlcInt modulo,
               IlcInt consistency) :
  IlcConstraintI(solver),
  modulo_(modulo),
  x_(x),
  consistency_(consistency) {
  ;
}


IlcModuloI::~IlcModuloI() {
  ;
}


void IlcModuloI::propagate() {
  switch (consistency_) {
  case BOUNDS_CONSISTENCY: {
    while ((x_.getMin() % modulo_ != 0) ||
       (x_.getMax() % modulo_ != 0)) {
      if (x_.getMin() % modulo_ != 0) {
    x_.setMin(x_.getMin()+1);
    continue;
      }
      if (x_.getMax() % modulo_ != 0) {
    std::cout << "Min/max values: " << x_.getMin() << "/" << x_.getMax() << std::endl;
    std::cout << "Decreasing maximum value by 1." << std::endl;
    x_.setMax(x_.getMax()-1);
    std::cout << "Min/max values: " << x_.getMin() << "/" << x_.getMax() << std::endl;
    std::cout << "------------------------------" << std::endl;
    continue;
      }
    }
    break;
  }

  case DOMAIN_CONSISTENCY: {
    IlcInt threshold = x_.getMin();
    while (threshold <= x_.getMax()) {
      if (threshold % modulo_ != 0) {
    x_.removeValue(threshold);
      }
      if (threshold == x_.getMax()) {
    break;
      }
      threshold = x_.getNextHigher(threshold);
    }
    break;
  }
  }
}


void IlcModuloI::post() {
  switch (consistency_) {
  case BOUNDS_CONSISTENCY: {
    x_.whenRange(this);
    break;
  }

  case DOMAIN_CONSISTENCY: {
    x_.whenDomain(this);
    break;
  }
  }
}


ILOCPCONSTRAINTWRAPPER3(IloModuloWrapper, solver,
            IloIntVar, x_,
            IloInt, modulo_,
            IloInt, consistency_) {
  use(solver, x_);
  return new (solver.getHeap()) IlcModuloI(solver,
                       solver.getIntVar(x_),
                       modulo_,
                       consistency_);
}


IloConstraint IloModulo(IloEnv env,
            IloIntVar x,
            IloInt modulo,
            IloInt consistency) {
  return IloModuloWrapper(env, x, modulo, consistency);
}

Answer 1

Indeed it really seems to be an issue with the propagation of user defined constraints in some very specific conditions. I'm not sure if there is a direct link with the number of workers. The development team of CP Optimizer will investigate this problem and we will let you know.