signal 11 with infeasible LPs

[keithbriggs]

I am getting a python crash with

Command terminated by signal 11

whenever I solve an infeasible LP problem. Actually, the solution appears to complete correctly, but the crash occurs when I try to return from my function. I tried lots of things, like deleting my model with

del scip

but that just triggers the crash anyway. So it must be caused by some internal memory mismanagement within pyscipopt.

The exact same code works perfectly when the LP is feasible.

[Joao-Dionisio]

Hey @keithbriggs ! Can you please share a minimal reproducible example?

[keithbriggs]

Not easy - it's in the middle of a large code.

[Joao-Dionisio]

Can you then please tell me what you're trying to do with the LP you get, and potentially also share the LP?

[keithbriggs]

I just want to determine feasibility, That is correctly being determined, but I can't return from my function. i should have said that I am using vtype='B'.

[Joao-Dionisio]

Alright. Maybe the issue is the same that PR #1161 is fixing, of variables and constraints not being properly freed upon model deletion. Assuming this is indeed the fix, it will only be properly available via PyPi once we add a new release, only after SCIP has a new minor release (shouldn't take more than a month). So, unfortunately, you would have to compile PySCIPOpt yourself.
Local compilation of PySCIPOpt is done with

git clone https://github.com/scipopt/PySCIPOpt.git
cd PySCIPOpt
git checkout fix-freeTransform-segfault
export SCIPOPTDIR=your/SCIP/binary/directory # if not installed globally. if needed, you can download a precompiled packaged from https://www.scipopt.org/index.php#download
pip install .

Also, if feasibility is only what you're after, it can be more performant to use the following:

from pyscipopt import Model, SCIP_PARAMEMPHASIS

scip = Model()
scip.setEmphasis(SCIP_PARAMEMPHASIS.FEASIBILITY)

Please let me know if this fixes things, or if you're having problems with the compilation.

[keithbriggs]

I was already doing SCIP_PARAMEMPHASIS.FEASIBILITY. I will try the local compile.

[keithbriggs]

Trying to install scipoptsuite_10.0.0-1+trixie_amd64.deb on my Ubuntu 24 system, I get:
scipoptsuite : Depends: libopenmpi40 (>= 5.0.7) but it is not installable

[Joao-Dionisio]

Strange... Maybe the easiest is to just do

  wget https://github.com/scipopt/scipoptsuite-deploy/releases/download/v0.10.0/libscip-linux.zip                                                                                                               
  unzip libscip-linux.zip                                                                                                                                                                                       
  export SCIPOPTDIR=$(pwd)/scip_install    

[keithbriggs]

Thanks, that works and then the the pip install works.

But the signal 11 problem is not fixed. It happens as soon as I try to do any manipulations on the model, or try to return from my function. Making the model global, which should mean that it is not deleted upon function return, still doesn't fix it.

I made a small minimal example - but it did not show the problem. I probably have to make a bigger one.

[keithbriggs]

Ok, I found that the bug is triggered by calling getSolTime(sol) in the infeasible case. When I take out this call, everything works.

[Joao-Dionisio]

Ah that makes sense, but it should still be fixed on our end. Thank you for the issue! I'll create a patch soon.

Also, if you don't mind, I enjoyed browsing a bit through your website, quite interesting stuff :)