Hi. In your problem there are alternative LP optimal solutions (due to dual degeneracy). So any change in the environment (in your case, the --nomip option that likely changes preprocessing) can randomly yield a different LP solution at the root node.
Note that deciding whether there exist an optimal LP solution at the root node that is integer is NP-hard in general, i.e., there is no easy way to tell the LP solver to take an integer solution in case of ties. Matteo Inviato da iPad > Il giorno 15/gen/2014, alle ore 19:54, Marc Goetschalckx > <[email protected]> ha scritto: > > I have an binary model (all variables are binary) > For one particular instance, the LP relaxation generates a binary solution. > I checked this with the --nomip option and verified the solution is > completely binary. > The LP relaxation is solved first inside the MIP solver. > However, the branch and cut part of the MIP solver gets called and the > callback function gets called with the reason CUTGEN. > I list next the logfile of this run. Note that solution values of LP > relaxation and MIP are identical. > > 0: obj = 1.009800000e+004 infeas = 1.800e+003 (0) > 500: obj = 1.049500000e+004 infeas = 1.800e+002 (0) > * 988: obj = 1.063500000e+004 infeas = 0.000e+000 (0) > * 1000: obj = 1.054200000e+004 infeas = 0.000e+000 (0) > * 1500: obj = 8.642500000e+003 infeas = 3.151e-014 (0) > * 2000: obj = 5.304500000e+003 infeas = 1.226e-016 (0) > * 2500: obj = 3.092666667e+003 infeas = 4.914e-016 (0) > * 3000: obj = 1.524000000e+003 infeas = 2.498e-015 (0) > * 3385: obj = 1.041000000e+003 infeas = 0.000e+000 (0) > OPTIMAL SOLUTION FOUND > Integer optimization begins... > + 3385: mip = not found yet >= -inf (1; 0) > + 3401: >>>>> 1.041000000e+003 >= 1.041000000e+003 0.0% (2; 0) > + 3401: mip = 1.041000000e+003 >= tree is empty 0.0% (0; 3) > INTEGER OPTIMAL SOLUTION FOUND > Time used: 1.6 secs > Memory used: 10.4 Mb (10947806 bytes) > > Why is the branch and cut section of the MIP solver even called. > How can the unnecessary call to the cut generation routine be avoided? > > -- > Marc Goetschalckx > > > _______________________________________________ > Help-glpk mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/help-glpk _______________________________________________ Help-glpk mailing list [email protected] https://lists.gnu.org/mailman/listinfo/help-glpk
