I am using GAMS with CPLEX as the solver. I have a really strange problem. I have a MIP model with many binary vairables which was solved within 5 minutes.

All of a sudden - without having changed anything - the same model requires quite much time to be solved. I let the solver run for 5000 seconds, but still the problem was not solved (meaning that the Gap of the best found solution is still quite high).

Has someone experienced something similar? Or could someone think about a possible reason for that? I uploaded the node log file of that run. Maybe someone of you will see something interesting in it.

Do you think the I should let the model run for more than 5000 Seconds? The improvement for the last 3000 senconds were 0 % with regard to the gap (sett uploaded logfile). This is why I stronlgy doubt that a higher runtime will significantly improve the results.

GAMS/Cplex (in default mode) runs deterministically when executed on the identical model instance on the same machine. There are a few options that can make it behave non-deterministically (only with option thread<>1). But even slight changes (e.g. the order of variables and equations, or a coefficient slightly changed) can result in a different path of the algorithm and hence in different performance of the solver. This is known as performance variability. Here are a few good entries on this topic:

It seems that you try to set the mipemphasis option in your GAMS program. However, mipemphasis is a CPLEX solver option (https://www.gams.com/latest/docs/S_CPLEX.html#CPLEXmipemphasis) that needs to be specified via a solver option file and then you also need to instruct the solver to read that option file , e.g as follows:

then I created a file called cplex.opt and this file looks like this:

* CPLEX options file
mipemphasis 2

I changed the number but nothing seemed to have an effect on the results. I always get the same results (when specifying the maximal running time of the solver to 100). Do you think this is just a coincidence or did I made a mistake?