MIPGAP not closed

Answered

Amin Ahmadi

July 20, 2022 20:11

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Hello,

I am solving a location-transportation optimization problem (Gurobi-Python) that is not challenging with the small number of nodes according to the literature. 

 In order to optimize the solution, MIPGap should be set to zero rather than the default value. When I set MIPGap to 0, it reaches gap 0 very quickly, but it doesn't close it after 10000 seconds, which is unacceptable. MIPFOcuse, cuts, and Presolve have already been changed to different possible values, but the time has not improved. Could you please let me know if you have any suggestions? 

 

 

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• Official comment

  

  Simranjit Kaur

  • Gurobi Staff

  November 16, 2025 23:32

  This post is more than three years old. Some information may not be up to date. For current information, please check the Gurobi Documentation or Knowledge Base. If you need more help, please create a new post in the community forum, or try Gurobot, our chatbot interface offering instant, expert-level support.
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  Matthias Miltenberger

  • Gurobi Staff

  July 22, 2022 11:11

  Hi Amin,

  It can be very difficult to prove the optimality of a solution - even if you already have the solution but the dual bound is not quite there, yet. This is precisely the reason why Gurobi's default is not 0. For almost all applications, it is perfectly fine to work with an "optimal" solution that may not have been proven to be optimal with very high accuracy. Especially since in most cases, no new solutions will be found during that last phase of the solving process so the invested time can often be regarded as wasted.

  Of course, this is model-dependent and there may very well be use-cases for a MIPGap of 0. You should check the found solution and verify whether this is acceptable for your problem. Otherwise, you would either need to invest more time to prove optimality or work on improving the formulation. Having a so-called strong formulation can be very beneficial in regards to the dual bound improvement. Please check out these video tutorial for more information on the topic:

  • Tech Talk – Converting Weak to Strong MIP Formulations - YouTube
  • Tech Talk - Converting Weak to Strong MIP Formulations Part 2 - YouTube

  Cheers,
  Matthias

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