Originally posted by: martin#r

Hi,

I  tested my model with various instances. Most of them work fine, but for a single instance I get:

CPLEX Error  1121: Can't crush solution form.

I already checked for possible solutions but the only suggestions I found were to disable dual and non-linear reduction, but CPLEX disables them automatically due to presence of my LazyConstraintCallback.

I only changed a few settings which are:

// only use a single thread
cplex->setParam( IloCplex::Threads, 1);
// traditional search
cplex->setParam( IloCplex::MIPSearch, CPX_MIPSEARCH_TRADITIONAL);

//use dual simplex
cplex->setParam( IloCplex::RootAlg, CPX_ALG_DUAL);
cplex->setParam( IloCplex::NodeAlg, CPX_ALG_DUAL);

and I disabled CPLEX cuts by:

cplex->setParam( IloCplex::FlowCovers, -1);
cplex->setParam( IloCplex::FlowPaths, -1);
cplex->setParam( IloCplex::MIRCuts, -1);     //disable mixed integer rounding cuts
cplex->setParam( IloCplex::FracCuts, -1);    //disable Gomory fractional cuts
cplex->setParam( IloCplex::ImplBd, -1);              //disable implied bound cuts
cplex->setParam( IloCplex::LiftProjCuts, -1);        //only relevant for cplex 12.6
cplex->setParam( IloCplex::EachCutLim, 0);
cplex->setParam( IloCplex::CutPass, 0);
cplex->setParam( IloCplex::FracPass, 0);

Furthermore, I use a lazy- and a user-callback.

Originally posted by: T_O

Does ist work, when you disable the complete presolver?

Originally posted by: martin#r

Good point. I wanted to mention that but then forgot about it. When I set:

cplex->setParam( IloCplex::PreInd, false);
cplex->setParam( IloCplex::RelaxPreInd, 0);
cplex->setParam( IloCplex::RepeatPresolve, 0);
cplex->setParam( IloCplex::PreslvNd, -1);
cplex->setParam( IloCplex::Probe, -1);
cplex->setParam( IloCplex::ProbeTime, 1e+75);

everything works fine. 

Disabling presolve completely should help but is probably not what you like since it may hurt performance. Do you get this error when you try to add a cut or a lazy constraint? When you only get it when trying to add a cut then it might be an option to just ignore that single cut. This will not affect correctness but may affect performance. If you get this when adding a lazy constraint then ignoring the constraint is of course not an option.

Could you post your model and the offending cut/constraint here? We could then try to figure out which presolve reduction you need to disable in order to allow CPLEX to crush this particular cut/constraint.

Also, what version of CPLEX are you using?

Originally posted by: martin#r

Thank you for the files. I downloaded them and managed to reproduce the problem here.

Unfortunately, at the moment this looks like a real bug in CPLEX, the only robust way to work around this I see at the moment is to disable full presolve :-(

Are the constraints for which you get the exception always of the same type? If they are, would it be an option to add them beforehand via IloCplex::addLazyConstraints() and see if that fixes the problem?

Originally posted by: martin#r

I am testing with a lot of instances but I get this exception only for a single instance. I am not shure what you mean with 'same type' but the constraints I add with the callback are instance-specific and might cause trouble for different instances.

What I meant that maybe you are separating different "classes" of constraints. Like

 class A: sum (i in I) x[i] <= a

 class B: sum (j in J) y[j] <= b

If you get the error only for class A constraints then you could add those constraints directly to CPLEX before starting the solve and separate only the constraints of class B.