PrePasses setting doesn't seem to have an effect?

Awaiting user input

Sebastian Ochmann

• September 07, 2021 14:35

Hello,

I'm trying to limit the time spent in presolve since it often takes minutes. However, even setting PrePasses = 1 doesn't seem to help at all. Does each line of output during presolve ("Presolve removed ... rows ...") correspond to a "pass"? If so, I still get multiple lines of output during presolve even when setting PrePasses = 1. Other settings I currently use:

Method = 1
MIPFocus = 2
Presolve = 1 (but also tried -1)
Cuts = 1
Disconnected = -1
Heuristics = 0.001

Is it just the complexity of the model that even PrePasses = 1 spends so much time in presolve or is there something else wrong?

Gurobi version: 9.1.2 on Linux, C++

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  Matthias Miltenberger

  Gurobi Staff

  • September 09, 2021 11:05

  Hi Sebastian,

  The parameter PrePasses actually limits the maximum number of passes through all the different presolving routines. It can still happen that there is a specific presolving routine that takes a lot of time.

  Gurobi tries to print a new line every 5 seconds - this does not relate to the actual passes through the presolving phase.

  Usually, the time invested in presolving pays off in a much faster-to-solve problem for the often more time-consuming branch-and-cut process.

  Did you try disabling Presolving altogether (Presolve=0) to see how the overall performance is affected?

  Cheers,
  Matthias

   

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