Continued there: Forum: CPLEX Optimizers pseudo-mip much faster than continous lp

Have an lp which is originally not a mip. when solving with simplex or network (using cplex), it takes about 1 Minute. When adding dummy integer 0..1, then cplex treats it as mip, solves the root-relaxation within a few seconds, and because solving the mip then is trivial, the problem is solved this way very fast. This is almost always the case. So my question is: How does mip parametrize the root-relaxation, so that it is so much faster? I use both for mip root-algo and the continous problem the same solver (e.g. network, or simplex), so I would assume the root-relaxation in mip and the solution of the continous case should take the same time. Or does mip use (some internal) callback of the network/simplex-solver to stop the relaxation, when the solution is "good enough"? From where does it know then what's good enough (know the theoretical objValue)?

asked 16 Jul '15, 16:23

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pbhd
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accept rate: 0%

retagged 22 Nov '15, 23:24

Rob%20Pratt's gravatar image

Rob Pratt
1.2k26

Very peculiar that it is faster so solve the LP as a MIP problem. It should not make a difference. You are probably better of asking this question on the CPLEX user forums. Something you can try is enabling/disabling presolve to see whether that makes a difference.

(16 Jul '15, 21:25) Joris Kinable

You can find a list of OR software forums at https://www.or-exchange.org/questions/9068/or-software-forums

(17 Jul '15, 05:06) 4er

Thanks for the tip. "Moved" it to the IBM-Forum CPLEX Optimizers. So this here could be deleted?

(17 Jul '15, 13:00) pbhd

There is no need to delete the question. Just put the link to that question here for future reference.

(17 Jul '15, 14:45) Ehsan ♦

The question has been closed for the following reason "Moved to alternative forum." by Paul Rubin 18 Jul '15, 15:44

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Asked: 16 Jul '15, 16:23

Seen: 674 times

Last updated: 22 Nov '15, 23:24

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