[SCIP] Doubt of SCIP

[Stefan Vigerske]

Hi,

via Pyomo, you have very limited access to SCIP.
Pyomo writes your problem into an .nl file and then calls the SCIP 
executable for this .nl file (using SCIP's AMPL interface).
You don't have easy control about what is happening while this 
executable is running. I guess if you get access to the log while SCIP 
is running, you could parse that and send an interrupt signal to the 
executable to make SCIP stop and return the best solution found so far.

If you connect to SCIP via an interface that uses SCIP as a library, 
then you have much more control.
For the first question: You can do this. For example, you could add an 
event handler, which would be called at every node, for example, and in 
the event handler you could call the SCIPinterruptSolve() function 
(https://www.scipopt.org/doc-8.0.2/html/group__PublicSolveMethods.php#gab207a6210ea634cca6fae72dde4d9a91) 
if you want SCIP to stop as soon as possible.
Also have a separate thread that monitors the state of SCIP and calls 
SCIPinterruptSolve() when appropriate should work.
For the second question: Some parameters can be changed in the middle of 
calculation. I don't think it is documented for which one this is the 
case, but limits like gap, time, solutions, should be possible. You can 
change parameters via an API (similar to how to call 
SCIPinterruptSolve() above) or via the SCIP shell (there is a section on 
this at https://www.scipopt.org/doc-8.0.2/html/SHELL.php).

Not all interfaces/APIs to SCIP 
(https://www.scipopt.org/doc-8.0.2/html/INTERFACES.php) offer the same 
amount of functionality. With the C API, you get everything. Since you 
say Pyomo, looking at PySCIPOpt is likely. It's likely that you have 
access to SCIPinterruptSolve() there, too. The PySCIPOpt devs/users 
could say for sure.

You should also be able to use Pyomo for the modeling and let it write 
out a .nl file, and then call SCIP via one of its APIs. You can then let 
SCIP read the .nl file and do the solve. Not sure about getting the 
solution back into Pyomo, though.

Hope that helps,
Stefan


On 23/11/2022 15:24, Pablo Falcón wrote:
> Hi,
> 
> I'm Pablo and I have two questions to see if SCIP is useful for me:
> 
>     - ¿Can I stop SCIP when I'm doing the calculation and get the results? I
>     know that I can establish parameters like time limit, the gap... I want to
>     stop it while calculating because in this way I see the results and I have
>     enough information to know if I want to stop the model.
>     - ¿Can I change parameters like the gap in the middle of a calculation?
>     For example, sometimes I establish a very low gap and the calculation
>     doesn't stop in a reasonable time.
> 
> I would like to know if I can apply these options to SCIP in Pyomo and if
> the answer is no if I can apply in another language.
> 
> Best regards,
> Pablo
> 
> 
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