Interpretation of GAP in CPLEX


This is a part of the engine-log output that I get from a small-scale mixed integer linear optimization problem that I solved in CPLEX 12.7.0

Nodes Cuts/ Node Left Objective IInf Best Integer Best Bound ItCnt Gap 0 0 280.0338 78 280.0338 72 0 0 428.8558 28 Cuts: 89 137 0 0 429.5221 34 Cuts: 2 142 0 0 429.7745 34 MIRcuts: 2 143 * 0+ 0 460.9166 429.7745 6.76% 0 2 429.7745 34 460.9166 429.8666 143 6.74% Elapsed time = 0.49 sec. (31.07 ticks, tree = 0.01 MB, solutions = 1) * 35 8 integral 0 438.1448 435.6381 211 0.57% Cover cuts applied: 17 Implied bound cuts applied: 10 Flow cuts applied: 11 Mixed integer rounding cuts applied: 9 Gomory fractional cuts applied: 24 Root node processing (before b&c): Real time = 0.45 sec. (31.09 ticks) Sequential b&c: Real time = 0.08 sec. (20.80 ticks) ------------ Total (root+branch&cut) = 0.53 sec. (51.89 ticks)

What I understand from this, is that the best integer solution (for the objective function) found has the value of 438.1448, whereas the relaxed solution (non integer values) has the value of 435.6381 as best bound solution.

( 438.1448 / 435.6381 ) - 1 = 0.57% GAP

Does this mean that the solution still has that small gap, however it is proven to be the optimal solution? I had the (maybe wrong) idea that optimality is proven by a 0% gap.

I'm not sure how to interpret it correctly. Thanks for your help in advance.


Yes you are right. The optimality is proven if the upper bound and the lower bound evaluate the same value, i.e. CPLEX could prove an optimality gap of 0%.

Since CPLEX stops with a solution that has a gap of 0.57%, I would assume that you configured an MIP-gap <1%. If you are interested in a solution with proven optimal, you should change the MIPGap parameter to zero. See also <a href="https://www.ibm.com/support/knowledgecenter/en/SS9UKU_12.4.0/com.ibm.cplex.zos.help/Parameters/topics/EpGap.html" rel="nofollow">here</a>.


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