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<font face="monospace">Hi,<br>
I'm using SCIP with AMPL, and I have a large problem involving
thousands of variables and contraints.<br>
I found that I can get some good results <br>
1) by setting heuristics to aggressive and <br>
2) Dividing the objective function by a large number (let me say
10^12)<br>
<br>
With my surprise, the improvement in the primal bound that is
shown during the calculation is lost as I print the OF value once
I reach the time limit. I cannot understand why this happens.
Furthermore, this does not happen if I divide the OF by a smaller
number (i.e. 10^6). Unfortunately, in the latter case, I cannot
get the same improvement in the solution.<br>
<br>
Could you please help me? <br>
Is there any option to print the best solution found during the
calculation?<br>
Or to store the whole solution pool?<br>
I found the option gams/dumpsolutions for GAMS, but I'm using
AMPL...<br>
<br>
Thank you,<br>
</font>
<div class="moz-signature">
<pre>Oreste Fecarotta
PhD in Hydraulic Engineering
Assistant Professor in Hydraulic Engineering
University of Naples "Federico II"
Via Claudio 21, 80125 Napoli (ITALY)
- ScopusID: <a href="https://www.scopus.com/authid/detail.uri?authorId=36503779200">36503779200</a>
- ORCID: <a href="https://orcid.org/0000-0003-0471-8821">0000-0003-0471-8821</a>
- WoSID: <a href="https://publons.com/researcher/1354138/oreste-fecarotta/">P-7250-2016</a>
- web: <a href="www.docenti.unina.it/oreste.fecarotta">Docenti unina</a>
- Ord.Ing.NA Sez.A 20328
- tel: +39-081-7683462
- fax: +39-081-1683456
</pre>
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<div class="moz-cite-prefix">Il 20/01/2021 09:31, Franzen, Christian
ha scritto:<br>
</div>
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cite="mid:e35df79bf4c24fb7b0088d0b34602ae0@or.rwth-aachen.de">
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<p>Hi Ambros.</p>
<p><br>
</p>
<p>Thanks for your reponse and sorry for answering late. I have
"switched off" presolving by including no presolver plugins,
hence I only got the presolving code from the propagators and
constraint handlers. That seems to work properly. I started to
include one presolver plugin after the other and there seems
to be a problem with "domcol". It fixed a lot of variables and
afterwards my problem is reported to be unbounded. I did not
test all presolver plugins yet, but I will try to test all of
them by the end of this week. Any idea concerning "domcol"?</p>
<div><br>
</div>
Regards
<div>Christian<br>
<br>
<div style="color: rgb(0, 0, 0);">
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<div id="x_divRplyFwdMsg" dir="ltr"><font
style="font-size:11pt" face="Calibri, sans-serif"
color="#000000"><b>Von:</b> Scip
<a class="moz-txt-link-rfc2396E" href="mailto:scip-bounces@zib.de"><scip-bounces@zib.de></a> im Auftrag von Ambros
Gleixner <a class="moz-txt-link-rfc2396E" href="mailto:gleixner@zib.de"><gleixner@zib.de></a><br>
<b>Gesendet:</b> Mittwoch, 13. Januar 2021 21:15<br>
<b>An:</b> <a class="moz-txt-link-abbreviated" href="mailto:scip@zib.de">scip@zib.de</a><br>
<b>Betreff:</b> Re: [SCIP] After stopping pricing and
doing restart, model is becoming unbounded during
presolve?</font>
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<font size="2"><span style="font-size:10pt;">
<div class="PlainText">Hi Christian,<br>
<br>
Strange indeed. Have you tried to turn off
presolving? Does it still <br>
happen? Have you tried to successively activate
individual presolvers <br>
to see when it starts failing?<br>
<br>
Best,<br>
Ambros<br>
<br>
<br>
Am 04.01.21 um 17:10 schrieb Franzen, Christian:<br>
> Hi Scip Team.<br>
> <br>
> <br>
> Thanks to the help of Gregor, I managed to make
SCIP stop pricing after <br>
> some iterations and doing a restart on the
reduced master without <br>
> pricing. Unfortunately, I am facing some really
strang behaviour. <br>
> Sometime it happens, that my model (for that I
already have found a <br>
> feasible solution), is reported to be unbound. I
wrote the corresponding <br>
> LP files before and after presolving to disk and
I see that some <br>
> variable bounds vanished, which seems to be the
reason for the <br>
> unboundness. Furthermore, some of the objective
coefficients changed <br>
> their sign. Any ideas how or why that is
happening during presolving?<br>
> <br>
> <br>
> Regards<br>
> <br>
> Christian<br>
> <br>
> <br>
> P.S.: I attach the SCIP log for one of these
runs.<br>
> <br>
> <br>
> _______________________________________________<br>
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