Solver also favors the first possible machine: Job Scheduling Problem with alternatives

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S. K.

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Sep 9, 2021, 2:01:23 AM9/9/21

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Hey,

I am working on a Job Shop scheduling problem where the jobs can be planned on different machines. (22 machines, about 400 jobs)

Like a more complex version of this: Scheduling with transitions

I have several objectives that I am optimizing one by one, see Multi-objective optimization, e.g. fulfillment of the due date, efficient transition between jobs, sum of the start times, etc.

I have 2 different modi.

1) The machine the job can be produced on is chosen, so the jobs are only scheduled in time per machine.

2) The solver can choose between several possible machines per job.

(I also have constraints for an earliest possible start date per job and a few dependencies between some jobs to make it more complex...)

Overall everything works fine in the moment, the solver finds solutions.

I hint the 2 seconds option with the solution from the first one, so that the solver finds an initial solution faster. This also works.

But my problem is: The solver seems to favor the first machine from a pool of all possible machines for a job.

And I don't understand why. Because I have a lot of jobs and machines, I run the problem with 12 threads overnight for 7-8 hours and I am not even close to an optimal solution, but it seems all the feasible solutions I get after this time "start with oacking everything on the first possible machine". And the due date objective or the sum of all start times would definitely be better if some of the jobs would be on other machines.