Hello,
I am a masters student in a University. i was using Cplex to solve a normal maximization problem with 2 constraints. My problem is: My optimizer gets stuck while going through the pre-solve. I have tried waiting for hours to see if it moves past it but it doesn't, if there was a problem with my code, it would have showed error or something. Below you can see how much further the optimizer runs and then gets stuck. It doesn't even show any error. Please tell me, why is this happening.



Tried aggregator 1 time.
MIP Presolve eliminated 10 rows and 3830 columns.
MIP Presolve modified 28 coefficients.
Reduced MIP has 30 rows, 201 columns, and 592 nonzeros.
Reduced MIP has 201 binaries, 0 generals, 0 SOSs, and 0 indicators.
Presolve time = 0.00 sec. (3.93 ticks)
Probing fixed 4 vars, tightened 0 bounds.
Probing time = 0.00 sec. (0.58 ticks)

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Rifat Rahman
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#DecisionOptimization

The presence of the "Presolve time = ..." line suggests that the solver completed the presolve phase. Is this the entirety of the CPLEX log output? Also, what do you mean by "with 2 constraints"? You apparently started with 40 constraints (rows) and were down to 30 after presolving.

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Paul Rubin
Professor Emeritus
Michigan State University
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Can you repeat your experiment with parameter CPX_PARAM_MIPDISPLAY set to 4? This may give more information about where the process is stuck.

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Daniel Junglas
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Original Message:
Sent: Tue August 11, 2020 04:14 PM
From: Paul Rubin
Subject: Cplex Java gets stuck at presolve

The presence of the "Presolve time = ..." line suggests that the solver completed the presolve phase. Is this the entirety of the CPLEX log output? Also, what do you mean by "with 2 constraints"? You apparently started with 40 constraints (rows) and were down to 30 after presolving.

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Paul Rubin
Professor Emeritus
Michigan State University
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Hello, To answer your first question is Yes. This is all the output of my CPLEX, it doesn't go beyond this. On 2nd question, i misspoke, i had 1 equation that resulted into 40 rows of constraint

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Rifat Rahman
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Original Message:
Sent: Tue August 11, 2020 04:14 PM
From: Paul Rubin
Subject: Cplex Java gets stuck at presolve

The presence of the "Presolve time = ..." line suggests that the solver completed the presolve phase. Is this the entirety of the CPLEX log output? Also, what do you mean by "with 2 constraints"? You apparently started with 40 constraints (rows) and were down to 30 after presolving.

------------------------------
Paul Rubin
Professor Emeritus
Michigan State University
------------------------------