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Running time is longer when the problem size is decreased

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  • Jaromił Najman
    Gurobi Staff Gurobi Staff

    Hi Miranda,

    Unfortunately, one can usually not state a priori that model A will solve faster than model B. This also applies even if both models have the same structure/formulation. There can be many different reasons for this behavior. One possibility is that for model A Gurobi finds a feasible solution which leads to a certain path in the B&B tree which makes it converge fast. For a smaller model B of the same structure, Gurobi finds a feasible solution with different properties, which leads to the exploration of a different path in the B&B tree taking more time in the end.

    You could try running your models with multiple different Seed parameter values to see whether your models are prone to performance variability, i.e., that your models are prone to already small changes in the solution path. If you find out that different seeds have quite an effect on the solution process, you could try running Gurobi's tuning tool to find parameters which might not only speed up the solution process but also improve performance stability w.r.t. to different seeds.

    Best regards, 
    Jaromił

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