[SCIP] Different results between 8.0.3 and 8.1.0

[Peter Notebaert]

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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