I'm not sure what the log files would contain beyond what you have already shown. If you want to post the .lp file for the original problems (all groups in a single model), I can try to run it and see if the log results are the same.

Am I correct to assume that when you solve all groups at once, there are constraints that span multiple groups? If so, it would not be surprising that solving one group at a time would produce better results than solving all groups at once. As a simple example, suppose I have two jobs to do, an employee who can do one job at cost $1K, and a consultant who can do one job at cost $5M. If I find the optimal allocation of just the first job, it goes to the employee at cost $1K. The same is true if I just solve for the second job. When I solve for both jobs, the cost is $5,001,000, not $2K. So I have to ask how you can be sure that the all-groups solution is suboptimal?

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Paul Rubin

Professor Emeritus

Michigan State University

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Original Message:

Sent: Wed January 04, 2023 04:29 PM

From: Shah

Subject: Cplex not reaching optimal solution

If you approve , I can send the log/lp files for reference….

thanks

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Shah

Original Message:

Sent: Wed January 04, 2023 04:19 PM

From: Shah

Subject: Cplex not reaching optimal solution

Thanks Professor.

this is a maximization problem.

solution status and solution value is printed after calling the solve method.

yes it makes hard to understand the behavior as it says solution is optimal with objective value.

also I'm able to get the values of variables and majority of them is 0's wherein we would expect it to be 1's at optimality.

what could be done potentially to resolve this?

thanks

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Shah

Original Message:

Sent: Wed January 04, 2023 02:53 PM

From: Paul Rubin

Subject: Cplex not reaching optimal solution

Is this a maximization or minimization problem?

The first node log does seem to indicate that the full model is infeasible. Were the "solution status" and "solution value" lines written by your code and, if so, how did you get the values ("Optimal" and "1.1383008422684283E22")? Were you able to recover the alleged solution (get the values of the decision variables)?

------------------------------

Paul Rubin

Professor Emeritus

Michigan State University

Original Message:

Sent: Wed January 04, 2023 12:11 PM

From: Shah

Subject: Cplex not reaching optimal solution

Hi Professor,

The allocations are very poor. It's a leave allocation optimization of staffs of different groups. When ran individual groups, it gives desirable outcome but with all the groups we are getting minimal assignments which is weird.

I see 'infeasible' in the node log file (I showed above) under the objective column and it's exiting in 0.16 seconds. Can we conclude something from this?

the log definitely seems some issues..?

for a well run scenario earlier (for a group seperately), the log looks like below;

MIP Presolve eliminated 387 rows and 1675 columns.

MIP Presolve modified 869 coefficients.

Reduced MIP has 580 rows, 2344 columns, and 33186 nonzeros.

Presolve time = 0.07 sec.

Clique table members: 146.

MIP emphasis: balance optimality and feasibility.

MIP search method: dynamic search.

Parallel mode: deterministic, using up to 2 threads.

Root relaxation solution time = 0.02 sec.

Nodes Cuts/

Node Left Objective IInf Best Integer Best Node ItCnt Gap

* 0+ 0 3.08034e+13 3.39977e+23 626 ---

* 0+ 0 3.22124e+23 3.39977e+23 626 5.54%

* 0+ 0 3.32806e+23 3.39977e+23 626 2.15%

0 0 3.32881e+23 22 3.32806e+23 Cuts: 26 757 0.02%

Zero-half cuts applied: 1

Gomory fractional cuts applied: 2

Root node processing (before b&c):

Real time = 0.13

Parallel b&c, 2 threads:

Real time = 0.00

Sync time (average) = 0.00

Wait time (average) = 0.00

-------

Total (root+branch&cut) = 0.13 sec.

Solver Version = 11.0.0

Cplex status = OptimalTol

NumericalEmphasis = false

Solution status = Optimal

Solution value = 3.328061358153315E23

Thanks

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Shah

Original Message:

Sent: Wed January 04, 2023 11:53 AM

From: Paul Rubin

Subject: Cplex not reaching optimal solution

When you say "nowhere near optimum", do you mean that you know a feasible solution with a better objective value?

------------------------------

Paul Rubin

Professor Emeritus

Michigan State University

Original Message:

Sent: Wed January 04, 2023 07:40 AM

From: Shah

Subject: Cplex not reaching optimal solution

Hi All,

I am experiencing an issue with my cplex solution (MIP). The log reads as below;

Tried aggregator 2 times.

MIP Presolve eliminated 2193 rows and 11266 columns.

MIP Presolve modified 4839 coefficients.Aggregator did 2 substitutions.

Reduced MIP has 3282 rows, 34713 columns, and 505657 nonzeros.

Presolve time = 1.06 sec.

Clique table members: 2161.

MIP emphasis: balance optimality and feasibility.

MIP search method: dynamic search.

Parallel mode: deterministic, using up to 2 threads.

Root relaxation solution time = 0.04 sec.

Nodes Cuts/

Node Left Objective IInf Best Integer Best Node ItCnt Gap

0 0 infeasible 0

Root node processing (before b&c):

Real time = 0.16

Parallel b&c, 2 threads Real time = 0.00

Sync time (average) = 0.00

Wait time (average) = 0.00

-------

Total (root+branch&cut) = 0.16 sec.

Solution status = Optimal

Solution value = 1.1383008422684283E22

Although solution status says optimal. I see that its no where near optimum. Could you please share your views?

Thanks,

Shahul

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Shah

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#DecisionOptimization