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Concurrent Optimizer and callbacks

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7 comments

  • David Torres Sanchez
    Gurobi Staff Gurobi Staff

    Hi Thomas,

    I am not sure what you mean here. Could you provide your code example?
    Also, you may want to have a look the callback.py example and the callback codes page.

    Cheers, 
    David

    0
  • Thomas Leveringhaus
    Collaborator
    Investigator
    Gurobi-versary

    Hi David,

    I have a callback-function like this (in python):

    def writesolution_callback(model, where):
        if where == GRB.Callback.MIPSOL:
    [...] # processing of the solution and creation of the dictionary named dictionary
        with open("solution.json", "w") as outfile:
                json.dump(dictionary, outfile)

    This callback-function is called by gurobi:

    model._vars = model.getVars()
    model.optimize(writesolution_callback)

    With model.setParam(GRB.Param.ConcurrentMIP, 2)  I observed that the solution is only written to file (as wanted with the callback) if a new solution was found in Instance 0 of ConcurrentMIP. When the log says "Instance 1 was solved" the new solution is not written to file. It seems to me that callbacks are only called from Instance 0 - is that the wanted behaviour of Gurobi?

    Best regards

    Thomas

    0
  • Thomas Leveringhaus
    Collaborator
    Investigator
    Gurobi-versary

    Or maybe Callbacks are called from alle ConcurrentMIP-Instances but maybe identifying "if where == GRB.Callback.MIPSOL" doesn't work properly in different instances?

    0
  • David Torres Sanchez
    Gurobi Staff Gurobi Staff

    Hi Thomas,

    Sorry for the delay.

    Thanks for the extra info. Indeed as you mention in the first message callbacks are only registered for a single solve or instance (this solve uses multiple threads).

    If you want to explore an alternative maybe the Pool related parameters might be helpful: Finding Multiple Solutions.

    Cheers,
    David

    0
  • Thomas Leveringhaus
    Collaborator
    Investigator
    Gurobi-versary

    Hi David,

    thank you for your answer: Then it is a feature not a bug ;-)

    But that is a pity for me, because I can't save the new found solution to file during optimization.

    Is there a special reason why callbacks are only used in one instance? Do you fear race conditions or something like that? I would be interested in a scientific reason. Or is it just not yet implemented? Are there plans to enable callbacks (or at least some types of them) for all instances in ConcurrentMIP?

    Best regards

    Thomas

    0
  • David Torres Sanchez
    Gurobi Staff Gurobi Staff

    Hi Thomas,

    I think it is not yet implemented.
    I will relay this feature request to the developers.

    Cheers, 
    David

    0
  • Thomas Leveringhaus
    Collaborator
    Investigator
    Gurobi-versary

    Thank you David!

    0

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