[SCIP] Different results between 8.0.3 and 8.1.0

[Kamp, Dominik]

Hello Thiago,

this looks like a bug. Therefore, it will be very helpful, if you file a bug report with reproducing settings, problem, desired solution, and full version of the permuted 8.1.0 run. Then we can have a closer look.

You can file a bug report at either

https://www.scipopt.org/bugs.php

or

https://github.com/scipopt/scip/issues.

Thanks in advance!

Best regards,

Dominik

> Am 01.02.2024 um 18:04 schrieb Thiago NOVAES <thiago.novaes at princeps.com>:
> 
> To add more information, there are 2 problems. The “original” is a MIP and “permutation” is a version of original that I changed the order of some rows and columns.
> 
> The table below show the objective function in each version of SCIP.
> 
> <image001.png> Thiago
>  De : Thiago NOVAES 
> Envoyé : jeudi 1 février 2024 17:44
> À : 'Peter Notebaert' <mail at peno.be>; s schnug <sascha.schnug at gmail.com>
> Cc : scip at zib.de
> Objet : RE: [SCIP] Different results between 8.0.3 and 8.1.0
>  Ok, that makes sense.
> 
> But in another case, I found 72,313,267 as objective with 8.1.0 and 71,882,637 with 8.0.3.
> 
> How can I found the source of this difference?  Thiago,
>  De : Peter Notebaert <mail at peno.be> 
> Envoyé : jeudi 1 février 2024 15:56
> À : s schnug <sascha.schnug at gmail.com>
> Cc : Thiago NOVAES <thiago.novaes at princeps.com>; scip at zib.de
> Objet : Re: [SCIP] Different results between 8.0.3 and 8.1.0
>  Note that this is not only in scip but for any solver. 
> Also the order in which the data is supplied can result in a different solution vector with the same objective value. For example providing the model via an lp model or via mps will most probably also have this effect.
>  Peter
>  On Thu, 1 Feb 2024 at 14:09, s schnug <sascha.schnug at gmail.com> wrote:
> Hi,
>  from the perspective of the solver, it only matters to find an optimal (objective) solution which is feasible (given some tolerances).
> Which solution of those candidates is picked is not guaranteed (although deterministic).
>  What you describe is completely natural!
> Every single change (e.g. heuristic priority weights; new heuristics) might lead to different result-vectors for many people (if we assume proven optimal result-status with the same objective).
>  It's even worse: You might see different result-vectors by just changing your operating-system or hardware in general (cannot give guarantees about SCIP here).
> The mailing-list contained an interesting runs-different-during-simulation-in-valgrind example today!
>  See https://coral.ise.lehigh.edu/mip-2008/talks/danna.pdf for some "related" background.
>  You should never assume result-vectors being stable across solutions (and maybe OS/hardware).
> Sometimes, there is some additional modelling possible, which can express your expectation about those side-constraints (e.g. lexicographic-ordering). But often this hurts.
>   Greetings,
> Sascha
>  (not a SCIP dev; but a happy user)
>  Am Do., 1. Feb. 2024 um 13:19 Uhr schrieb Thiago NOVAES <thiago.novaes at princeps.com>:
> Hello,
>  The result of a MIP (1 binary, 0 integer, 0 implicit integer, 2751 continuous, 2452 constraints) is different between versions 8.0.3 and 8.1.0. Difference only in the variables, not in the objective function.
> Looking at the release notes for 8.0.4 and 8.1.0 I didn't find anything that justified the difference.
>  Could anyone help me understand this better? Is there no guarantee on variables between versions? I can share the .lp if it helps.
>  Best regards,
> Thiago
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