running time to optimality

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Iason Liagkas

2023年06月05日 11:32

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Hello guys, I have been running a code that solves a problem but I do not know if the problem is feasible or not, so my questions are 1) if the log file has been running for 24 hours and I have not reached to any solution yet could it be because I have an infeasible problem? or since it did not crash in the beginning it means I will get a solution eventually? 2) the root relaxation objective value displayed above the log is not supposed to be the presolve solution right?  Another question is: is there any possibility that the code would run forever? 

Kind regards 
Iason

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  Mario Ruthmair

  • Gurobi Staff

  2023年06月05日 12:26

  Hi Iason,

  1) If the solver did not find a solution yet but it is still going on, then there is a chance that your model is infeasible. But the solver does not know yet. It could also be that finding a solution is very challenging. Proving infeasibility can be very difficult. If the model is actually infeasible and if presolving and the root relaxation did not lead to the proof of infeasibility, the branch-and-bound phase can take a very long time.

  2) The root relaxation is obtained by relaxing integrality on all integer variables. The objective value from the resulting relaxation gives a dual bound on the optimal value. So no, it is not a feasible solution for the original problem.

  3) The solver has a finite solution runtime for all models (provided that there is no bug), but the runtime can be very high, and you could potentially run into memory limits before the model is solved.

  Best regards,
  Mario

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