<br><font size=2 face="sans-serif">Michael,</font>
<br>
<br><font size=2 face="sans-serif">CglPreprocess can detect some integer
variables but is not very sophisticated. One can always use priorities
so that the extra variables are only used in cut generation and not in
branching.</font>
<br>
<br><font size=2 face="sans-serif">John</font>
<br>
<br>
<br>
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<td width=40%><font size=1 face="sans-serif"><b>Michael Hennebry <hennebry@web.cs.ndsu.nodak.edu></b>
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<br><font size=1 face="sans-serif">Sent by: coin-discuss-bounces@list.coin-or.org</font>
<p><font size=1 face="sans-serif">06/04/2006 02:27 AM</font>
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<div align=center><font size=1 face="sans-serif">Please respond to<br>
Discussions about open source software for Operations Research
<coin-discuss@list.coin-or.org></font></div></table>
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<div align=right><font size=1 face="sans-serif">To</font></div>
<td><font size=1 face="sans-serif">Discussions about open source software
for Operations Research <coin-discuss@list.coin-or.org></font>
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<div align=right><font size=1 face="sans-serif">cc</font></div>
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<div align=right><font size=1 face="sans-serif">Subject</font></div>
<td><font size=1 face="sans-serif">Re: [Coin-discuss] Suggestions on how
to go about tuning?</font></table>
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<br><tt><font size=2>On Sat, 3 Jun 2006, John J Forrest wrote:<br>
<br>
> The first thing to do is solve the problem however slowly and look
at log<br>
> and solution. Using defaults this took 1148 seconds and 150K
nodes.<br>
> Looking at the log file it looked as if strong branching was not buying<br>
> very much. What was more interesting was that all variables
took integer<br>
> values although only half were declared integer. Assuming this
would<br>
> always be true I tried setting them all integer. This time strong<br>
> branching was kicking in and it took 5K nodes and 413 seconds. I
also saw<br>
> that cuts and heuristics were not doing much good so switched them
off.<br>
<br>
It occurs to me that sometimes it might be important to be able<br>
to distinguish between constraints that define the problem<br>
and constraints that can be inferred or can be accepted.<br>
<br>
In the case at hand, at least one solver runs faster and gets<br>
the right answer with all variables specified as integer.<br>
<br>
'Twouldn't be a surprise if some solver ran faster with<br>
only the original integer variables declared integer.<br>
<br>
A solver that knew which kind it was could accept or reject<br>
constraints that didn't define the problem accordingly.<br>
Better yet, it might use a strategy that enforces or relaxes<br>
a non-problem-defining constraint whichever is more efficient.<br>
<br>
In the case at hand, a solver might employ a rounding heuristic<br>
that did not enforce integrality on all the variables,<br>
but it might nevertheless use Gomory fractional cuts,<br>
which require integer variables.<br>
<br>
-- <br>
Mike hennebry@web.cs.ndsu.NoDak.edu<br>
"it stands to reason that they weren't always called the ancients."<br>
-- Daniel Jackson<br>
<br>
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