[Scip] scaling

[Thorsten Koch]

Hi Tamas,

when called from SCIP the SoPlex scaler is not invoked.
This is on our todo list, expect it to be in the next release ;-)

The people working on it are currently traveling, we will have a look
at it and notify you once we have fixed it.

Best reagards
Thorsten


Tobias Achterberg schrieb:
> 
> Mahr Tamas wrote:
>> Thank you for looking into this!
>>
>> I wonder if the problem is really at soplex. I have tried to solve the 
>> same problem with the soplex shell command, and it can solve it very 
>> quickly. (I guess it solves the lp relaxation of the mip, but I am 
>> guessing here. In fact I have no clue how soplex interprets a mip.)
>>
>> Looking into the soplex code reveals that their cli program uses the 
>> MySoPlex class that is derived from the SoPlex class. The scip interface 
>> class for soplex (SPxSCIP), however, is derived from a different line, 
>> namely the SPxSolver class. This class is a sibling of the SoPlex class. 
>> Unfortunately, the SPxSolver class do not use the presolver or scaler 
>> classes that are used in the SoPlex class. That is why the command line 
>> program can solve this problem, while the scip solver cannot.
>>
>> I think either the SPxSCIP class should be derived from the SoPlex class 
>> and then it could make use of the scaling/presolving classes in there, 
>> or the SPxSCIP class could at least mimic the SoPlex class by applying 
>> the presolving/scaling classes in a similat manner. But maybe I am 
>> totally wrong here, and scaling/presolving should happen already in scip 
>> at the beginning of the branch-and-bound search and not in soplex every 
>> time an lp relaxation is solved.
> 
> No, you are completely right. The SoPlex class is basically a wrapper around the core 
> SPxSolver class to provide presolving and scaling.
> 
> However, this is not as easy as it seems. I don't know whether SoPlex already supports all 
> the calls that are needed for SCIP, for example to access the simplex tableau rows (and 
> automatically unscale it).
> 
> 
> Tobias
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-- 
The important thing is not to stop questioning.
Curiosity has its own reason for existing.          -- Albert Einstein
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Dr. Thorsten Koch / Konrad-Zuse-Zentrum für Informationstechnik Berlin
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