I have a power systems optimization model. It is pretty large model, but Gurobi did not have any problem to solve it in 4 hours. This was the situation a few weeks ago. Below, you can see the number of variables and constraints before and after pre-solve procedure.
I recently added a few decision variables (126 continuous and 42 binary) and constraints. Gurobi can not solve this new model in 22 hours. Below, you can see the number of variables and constraints before and after pre-solve.
So, previously, there were 1,541,716 continuous decision variables before pre-solve. I added 126 continuous variables and total number of continuous decision variables turned out to be 1,541,842 in new model. This is correct. Similarly, there were 369,640 integer variables. I added 42 binary variables to the model and total number of integer variables turned out to be 369,682. This is correct too.
What I do not understand is the big difference in number of integer variables after pre-solve in two models. Previously, there were 251,215 integer variables after pre-solve. In the new model, this number is 369,115. Why is this happening? I assume that this is the reason why Gurobi can not solve the recent model in a reasonable amount of time. Again, previous model was solved by Gurobi in 4 hours. However, the recent model can not be solved in 22 hours. 22 hours is the time limit I put for Gurobi. All 22 hours were spent for root relaxation. I do not event see the log for branch and bound tree.
Can you please help me with this?
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