Forum Discussion
How to detect between termination by timeout or tolerance
The documentation doesn't clearly explain XPRS_UNF. I believe it means the optimisation finished either due to a XPRS_MAXTIME being reached or a tolerance being met. Without comparing time and tolera...
Hi Colin.
Most of the stopping criteria state that a user-imposed working limit has been hit: TIMELIMIT, SOLLIMIT, NODELIMIT, ITERLIMIT, MIPGAP
for limits on the execution time, the maximum number of solutions to be found, the maximum number of nodes in the search tree to be explored, the maximum number of simplex iterations to be conducted, and the limit for the primal-dual gap before the optimization can be stopped, respectively.
The next common thing are errors like MEMORYERROR, NUMERICALERROR, and LICENSELOST.
You are right that SOLVECOMPLETE is a bit of a special case: It indicates that the problem could be solved to proven optimality, without needing to explore the full search tree, typically by the solver detecting some globally valid information that proves that no improving solution can be found anymore.
As two examples, consider:
a) When the MIP problem decomposes into a set of completely(!) independent subproblems, Xpress will solve each of them individually. If one of those subproblems is found to be infeasible, then the overall problem is infeasible. This can be concluded without finishing the other problems (or even attempting to do so).
b) The solver will generate new valid constraints throughout the search, so-called cutting planes. If one such globally valid(!) constraint can be reduced to a false statement such as 1 <= 0, this mathematically proves that the remaining search problem is empty and the current incumbent solution is optimal.