Strange behavior - Optimal solutions with different values, depend on whether StartSol is included

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Quan Hoang

2024年06月17日 08:54 編集済み

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Hello Gurobi support team,

I'm building an MILP model by reading from a .mps file but a strange behavior has occurred: the optimal value depends on whether a .sol file is included.

Below is the code to generate the observation. I'm willing to upload the .mps and .sol files, but unfortunately, I can not figure out how to do so.

import gurobipy as gp
from gurobipy import GRB

modelWithStartSol = gp.read('model_MILP.mps')
modelWithStartSol.read('model_MILP.sol')
modelWithStartSol.optimize()
assert modelWithStartSol.status is GRB.OPTIMAL

modelWithoutStartSol = gp.read('model_MILP.mps')
modelWithoutStartSol.optimize()
assert modelWithoutStartSol.status is GRB.OPTIMAL

print(f'{modelWithStartSol.objVal = }') # 162720.0
print(f'{modelWithoutStartSol.objVal = }') # 162780.0

I'm looking forward for your response.

Best regards,

Quan.

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• 正式なコメント

  

  Simranjit Kaur

  • Gurobi Staff

  2026年11月13日 01:23

  This post is more than two 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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  Jaromił Najman

  • Gurobi Staff

  2024年06月13日 12:11

  Hi Quan,

  Note that uploading files in the Community Forum is not possible but we discuss an alternative in Posting to the Community Forum.

  Best regards, 
  Jaromił

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  Quan Hoang

  2024年06月13日 16:35

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  Dear Jaromił,

  thank you for the instruction.

  Here is the Google Drive link to the two files I mentioned in the previous post. https://drive.google.com/drive/folders/1SqJGWgwyW8znC0RCoBkg6-nLHdhTl-G_?usp=sharing

  Hope it helps.

  Best regards,

  Quan.

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  Jaromił Najman

  • Gurobi Staff

  2024年06月14日 07:49

  Hi Quan,

  Thank you for sharing the model and the solution file. I can reproduce the behavior and will investigate. I will update you on my findings.

  Best regards, 
  Jaromił

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  Jaromił Najman

  • Gurobi Staff

  2024年06月17日 07:31

  Hi Quan,

  Thank you for reporting the issue.

  We have found the source of the issue and a fix will be available with the upcoming technical release.

  Best regards, 
  Jaromił

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  Quan Hoang

  2024年06月17日 07:56

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  Hi Jaromił,

  Thank you for updating me on the issue. I'll make sure to use the next release once it's available.

  In the meantime, is there any workaround to obtain the correct solution with the current Gurobi release (11.0.2)? I'm thinking of some (tricky) parameter settings but not entirely sure.

  Best regards,

  Quan.

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  Jaromił Najman

  • Gurobi Staff

  2024年06月17日 08:01

  Hi Quan,

  Unfortunately, the only official parameter to currently avoid this issue is to set Presolve=0.

  Best regards, 
  Jaromił

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  Quan Hoang

  2024年06月17日 08:58

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  Thanks for letting me know. That is indeed unfortunate.

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