# Minimizing Changeover Time in Scheduling Problem - Computation time

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Hello,

At the moment I am performing a scheduling problem with time windows.
The model is very similar to the second model which can be downloaded from the AIMMS website (see link) execpt from a number of elements:
• I try to minimize the sum of all the changeovers that are used.
• Base unit is in minutes.
In total I have 3 datasets (one for each machine) that AIMMS can read in from an Excelfile, one at a time.
Initially, my model is able to solve using CPOptimizer 12.8 for all machines, where each job has to be scheduled after the "StartTime" and finish before an "Endtime".
After this solution I would like to decrease the StartTime by one week when a job meets a certain criteria and then solve it again.
When I run the procedure that decreases the Startime by one week and try to solve it again it finds a solution for only one of the machines and for the other two AIMMS hasnt been able to find a solution yet after more than 9 hours.
For the machine that could find a solution I repeated the procedure (decrease Startime by another week & Solve it). This did not find a solution after 27 hours.

Now my question is can I do something about this computation time, since the computation time increases and AIMMS is not yet able to find a solution when I decrease the StartTime by one week. Ultimately, I want to decrease the StartTime 3 weeks when jobs meet a the criteria.

If you need any extra information, please let me know!

Kind regards,
Stefan

https://how-to.aimms.com/Articles/142/142-Narrowing-Time-Windows.html

### 4 replies

Userlevel 5
+2
Hello @stefan1991 ,

Say, you have a job with Start Time = Week 3 and End Time = Week 6, it was scheduled from Week 4 to Week 5(run time of 1 week).

Now, do you want to start this job exactly in Week 3 (Week 4 - 1) or do you want to find a different solution where this job has Start Time = Week 2 (Week 3 -1 ) and End Time = Week 6 ?
Hi @mohansx,

For me, it does not really matter when a job is exactly scheduled. Every job has to be scheduled between its Startime and EndTime, but the main problem is that it does not yet find a solution.

Say, I have 20 jobs every week in a year. Each job has a StartTime and Endtime. In my first model each job has a StartTime = Week x and EndTime = Week x + 1. For this first model I/AIMMS is able to find a solution.

For the next model, for some jobs the Startime = Week x -1 and EndTime = Week x +1. The Jobs that have an earlier StartTime do not have to be scheduled earlier, but they could be. The main problem is that AIMMS does not yet find a solution after 27hours.

If you need any information, please let me know!

Stefan
Userlevel 6
+6
Hi @stefan1991 - sorry, it looks like we overlooked your last reply! Do you still need help with this, or did you find a solution?
Hi @Jess Es , I already found a solution. Thanks!