[Cbc] Why Cbc takes too long to solve MIP with integer LP relaxation?

[Lidio Meireles]

Dear All,

I have a MIP instance where the solution of the LP relaxation is
integer for all variables. This solution should also be the MIP
solution, right?

Cbc takes 4.7 seconds to solve the LP relaxation but after that it
takes 77.59 seconds to solve the MIP problem. For comparison, GLPK
takes 20 seconds to solve the LP relaxation and no time to solve this
particular MIP instance. Why is Cbc taking too long? Cbc stdout is
shown below. Thanks. Lidio


Coin Cbc and Clp Solver version 2.4.1, build May 26 2010
command line - cbc scpopt.lp (default strategy 1)
Continuous objective value is -501.609 - 4.70 seconds
Cgl0004I processed model has 17032 rows, 112313 columns (112313 integer) and 240
719 elements
Cbc0038I Solution found of -501.609
Cbc0038I Before mini branch and bound, 112313 integers at bound fixed
and 0 continuous
Cbc0038I Mini branch and bound did not improve solution (0.76 seconds)
Cbc0038I After 0.76 seconds - Feasibility pump exiting with objective of -501.60
9 - took 0.50 seconds
Cbc0012I Integer solution of -501.609 found by feasibility pump after 0 iteratio
ns and 0 nodes (0.82 seconds)
Cbc0038I Full problem 17032 rows 112313 columns, reduced to 0 rows 0 columns
Cbc0001I Search completed - best objective -501.6091222967904, took 0 iterations
 and 0 nodes (77.59 seconds)
Cbc0035I Maximum depth 0, 0 variables fixed on reduced cost
Cuts at root node changed objective from -501.609 to -501.609
Probing was tried 0 times and created 0 cuts of which 0 were active after adding
 rounds of cuts (0.000 seconds)
Gomory was tried 0 times and created 0 cuts of which 0 were active after adding
rounds of cuts (0.000 seconds)
Knapsack was tried 0 times and created 0 cuts of which 0 were active after addin
g rounds of cuts (0.000 seconds)
Clique was tried 0 times and created 0 cuts of which 0 were active after adding
rounds of cuts (0.000 seconds)
MixedIntegerRounding2 was tried 0 times and created 0 cuts of which 0 were activ
e after adding rounds of cuts (0.000 seconds)
FlowCover was tried 0 times and created 0 cuts of which 0 were active after addi
ng rounds of cuts (0.000 seconds)
TwoMirCuts was tried 0 times and created 0 cuts of which 0 were active after add
ing rounds of cuts (0.000 seconds)
Result - Finished objective -501.6091222967904 after 0 nodes and 0 iterations -
took 83.28 seconds (total time 88.94)
Total time 89.82