Re: [Help-glpk] TRYING TO GET A NON-OPTMIAL SOLUTION OF A MIP MODEL.

[Joshua Friedman]

I believe glpk is too slow for timetable problems like yours. Try Gurobi. 

On Tue, Jun 12, 2018, 8:05 PM João Pedro de Sá Moreira <address@hidden> wrote:

I’m currently working on a timetable model that is supposed to deal if a pretty big number of variables.

 

On initial tests i can see it takes a fair time to get to a LP solution (about 30m) and then it starts to work on finding the optimal integer solution.

 

The problem is that in this part, it takes about 5h working on analyzing the whole tree, but the strange thing is that it keeps showing the same value every iteration it does until it gets to the end of the tree. The value it shows is the same value that can be seen when it finds the lp solution, does it means that the LP solution was the optimal since the beginning? Am i able to interrupt the execution and take a look at this solution? Or is there a way to speed it up a little bit?

 

I’ve alreay tried –tmlim and it worked but the solution couldn’t be returned. Then i tried –nopresol and –output but the problem is that the output returned a “INTEGER UNDEFINED” status.

 

That’s the part of the model where it finds the optmial LP solution:

 

Solving LP relaxation...

GLPK Simplex Optimizer, v4.65

15202 rows, 138924 columns, 555696 non-zeros

      0: obj =  -0.000000000e+00 inf =   3.194e+03 (620)

Perturbing LP to avoid stalling [401]...

   2722: obj =   8.750000000e+02 inf =   1.930e+02 (52) 2

   4678: obj =   8.951560756e+02 inf =   1.026e+01 (9) 11

   4915: obj =   9.000461789e+02 inf =   8.944e-06 (0) 1

*  6780: obj =   9.391853182e+02 inf =   7.886e-06 (49431) 11

Removing LP perturbation [8499]...

*  8499: obj =   9.680000000e+02 inf =   1.588e-14 (0) 10

OPTIMAL LP SOLUTION FOUND

 

Then it begins the integer optmization and keeps on working and 5 hours later:

 

.

.

.

+ 86591: mip =     not found yet <=   9.680000000e+02        (1184; 0)

+ 86616: mip =     not found yet <=   9.680000000e+02        (1187; 0)

+ 86651: mip =     not found yet <=   9.680000000e+02        (1191; 0)

+ 86727: mip =     not found yet <=   9.680000000e+02        (1195; 0)

+ 86818: mip =     not found yet <=   9.680000000e+02        (1205; 0)

Time used: 18130.5 secs.  Memory used: 313.8 Mb.

+ 86874: >>>>>   9.680000000e+02 <=   9.680000000e+02   0.0% (1213; 0)

+ 86874: mip =   9.680000000e+02 <=     tree is empty   0.0% (0; 2425)

INTEGER OPTIMAL SOLUTION FOUND

Time used:   18154.2 secs

Memory used: 413.6 Mb (433691909 bytes)

 

Then the timetable is shown.

 

Any tips? Thanks in advance.

 

 

 

 

 

 

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