[Symphony] Unexpected behaviour of gap_limit in symphony 5.1

[Pierre.Lebodic at lri.fr]

Hello,

I mentioned earlier a "bug" involving gap_limit in SYMPHONY 5.1.10. It
should be possible to reproduce it with the following test instance:
http://www.lri.fr/~lebodicp/wp-content/uploads/gap_instance.MPS

In the following, cuts are disabled to prevent possible numerical surprises.
In SYMPHONY 5.1.10, with gap_limit set to 5 (for instance), the following
happens:
****************************************************
* Optimal Solution Found                           *
* Now displaying stats and best solution found...  *
****************************************************
...
Current Upper Bound:         2.304
Current Lower Bound:         1.300
Gap Percentage:              43.58

Solution Found: Node 0, Level 0
Solution Cost: 2.304

and a feasible solution is displayed. Although the gap percentage is
43.58, the solver indicates that the optimal has been found.

In SYMPHONY 5.1.10, with gap_limit set to -1, a better solution is provided:
****************************************************
* Optimal Solution Found                           *
* Now displaying stats and best solution found...  *
****************************************************
...
Solution Found: Node 5, Level 3
Solution Cost: 2.062

and the optimal solution is displayed.


SYMPHONY 5.2 seems to work: with gap_limit lower than 43.58, the optimal
solution is found. with gap_limit set higher than 43.58, it provides the
same feasible solution as SYMPHONY 5.1.10.
Thanks for your help.

Cheers,

Pierre