Originally posted by: pnuzyf

Hi,

The attached is my test model! When the NumChan is set to 11, Nic is set to 12. It takes too long time to execute and finally "out of memory" comes out. In fact, the number of variables and constraints is not so large!

Could you tell me why? Thanks in advance!
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Originally posted by: SystemAdmin

For MIP the number of variables/constraints is not a clear indicator of whether the problem is difficult or not. There are small problems that are ultra-hard to solve and there are very big problems that solve easily. After all, we are talking about NP hard problems here ...
What version of CPLEX do you use?
It seems like you are trying to use node files: I see you setting the workmem and workdir parameters. But I don't see you setting the NodeFileInd parameter anywhere. This must be set to 2 or 3 to actually write files to disk.
On my machine (CPLEX 12.4.0.1, 2 threads, 4 GB RAM) the problem solves within 13 minutes and without any memory issues:

Nodefile size = 256.85 MB (203.08 MB after compression)
  12475 10033        0.1167    90        0.1307        0.1089   794341   16.67%
  12758 10293        0.1277    78        0.1307        0.1089   802929   16.67%
* 12847+10327                            0.1280        0.1089   807003   14.93%
 
Clique cuts applied:  41
Cover cuts applied:  259
Flow cuts applied:  1
Mixed integer rounding cuts applied:  1
Zero-half cuts applied:  13
Gomory fractional cuts applied:  11
 
Root node processing (before b&c):
  Real time             =   27.94
Parallel b&c, 2 threads:
  Real time             =  712.33
  Sync time (average)   =    0.01
  Wait time (average)   =    0.00
                          -------
Total (root+branch&cut) =  740.26 sec.

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Originally posted by: pnuzyf

Thanks for your kind reply! What do you mean "nodefileInd" parameter? Is it the "Node storage file switch"? If it is, I have already set it to "node file on disk and compressed". Please check it again!

I use cplex 12.4 32bit version. My machine is 12 threads and 8G RAM?

What should I do?
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Originally posted by: pnuzyf

Is the problem solved within 13 minutes? But From your snapshot, I found the optimality gap is more than 10 percent?
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Originally posted by: SystemAdmin

True, I used a large gap limit.
I went back to that and used a smaller gap limit. With that I run into the same problem as you: I eventually run out of memory before optimality within the specified gap can be proven.
Is there a way you can improve your model formulation to get tighter bounds?
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