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<div class="moz-cite-prefix">Good morning Eleonora,<br>
<br>
<br>
Am 31.01.2017 um 10:03 schrieb Pippia Eleonora:<br>
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<div>Good morning,</div>
<div>I’m using SCIP to solve an ILP model MP in order to test a
conjecture related to graphs of low dimensions. Actually, the
conjecture is equivalent to state that an integer linear
programming model MP is unfeasible. I have few questions.</div>
<div><br>
</div>
<div>1) I’ve implemented two constraint handlers with separation
callbacks. I don’t understand in which order the callbacks
CONSSEPALP for the different constraints are called. Assume
that at a node of the branch-and-bound tree, four steps of the
separation loop take place. It happened that in the first
three steps SCIP recalled the SEPALP callback from the first
constraint handler to add a new violated cut so that the
SEPALP from the second constraint handler was delayed in these
steps. At the fourth step, sometimes SCIP called immediately
the SEPALP from the second constraint handler without calling
the callback from the first one. Why can this happen?</div>
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from your question, I guess you already made yourself familiar with
the priorities to decide the order in which separators are called,
and the delay mechanism. <br>
There are some global parameters to further control the separation
process:<br>
<br>
<pre> maxcuts maximal number of cuts separated per separation round (0: disable local separation) [100]</pre>
<pre> maxcutsroot maximal number of separated cuts at the root node (0: disable root node separation) [2000]</pre>
<pre> maxroundsroot maximal number of separation rounds in the root node (-1: unlimited) [-1]</pre>
<pre> maxstallrounds maximal number of consecutive separation rounds without objective or integrality improvement (-1: no additional restriction) [5]</pre>
<br>
One reason for calling the delayed separator first might be that the
stalling rounds limit "maxstallrounds" is reached.<br>
If you would like to see a verbose explanation of what happens in
SCIP right before separator is called, you can <br>
recompile SCIP after putting<br>
<br>
#define SCIP_DEBUG<br>
<br>
at the top of the two files src/scip/{sepa,solve}.c. If this still
doesn't explain the behavior of the separators, please report back
to me.
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<div>2) Moreover, I ran the same instance with the same model MP
but on two different computers. It happened that at the same
node of the branch-and-bound, on the two computers SCIP found
two different fractional solutions of the LP. This had a
strong impact on the number of nodes generated to solve the
instance. Is it a normal behavior?</div>
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Unfortunately, the answer is yes. This is why we always report our
experimental setups as detailed as possible, including the operating
system, the used compiler and its version, and the processor
architecture plus memory size.<br>
In my experience, matching operating system version and compiler
version are sufficient in order to obtain reproducible results.<br>
<br>
Can you give more details on the architectures you are testing on?<br>
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<div>3) Finally, is there any way to use SCIP as an MIP-solver
disabling all the constraint programming features?</div>
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There is no such emphasis setting that comes with SCIP. I assume you
would like to test a pure branch-and-cut approach, but disable
restarts, conflict analysis, and domain propagation. However, I am
warning you that such features are integrated and very essential for
the solving process of all modern MIP solvers. Here is what a custom
SCIP settings file for this purpose might look like:<br>
<br>
<pre>conflict/enable = FALSE</pre>
<pre>presolving/maxrestarts = 0</pre>
<br>
In addition, set the propagation frequencies of all active
propagators and constraint handlers (propagation frequency >=0)
to -1 instead:<br>
<br>
<pre>>> grep -P "(propagating/.*/freq|constraints/.*/propfreq) = [01]" default.set</pre>
<pre>constraints/nonlinear/propfreq = 1</pre>
<pre>constraints/quadratic/propfreq = 1</pre>
<pre>constraints/linear/propfreq = 1</pre>
<pre>constraints/abspower/propfreq = 1</pre>
<pre>constraints/and/propfreq = 1</pre>
<pre>constraints/bivariate/propfreq = 1</pre>
<pre>constraints/bounddisjunction/propfreq = 1</pre>
<pre>constraints/cardinality/propfreq = 1</pre>
<pre>constraints/cumulative/propfreq = 1</pre>
<pre>constraints/indicator/propfreq = 1</pre>
<pre>constraints/knapsack/propfreq = 1</pre>
<pre>constraints/linking/propfreq = 1</pre>
<pre>constraints/logicor/propfreq = 1</pre>
<pre>constraints/or/propfreq = 1</pre>
<pre>constraints/setppc/propfreq = 1</pre>
<pre>constraints/soc/propfreq = 1</pre>
<pre>constraints/SOS1/propfreq = 1</pre>
<pre>constraints/SOS2/propfreq = 1</pre>
<pre>constraints/superindicator/propfreq = 1</pre>
<pre>constraints/varbound/propfreq = 1</pre>
<pre>constraints/xor/propfreq = 1</pre>
<pre>constraints/components/propfreq = 1</pre>
<pre>propagating/dualfix/freq = 0</pre>
<pre>propagating/genvbounds/freq = 1</pre>
<pre>propagating/obbt/freq = 0</pre>
<pre>propagating/pseudoobj/freq = 1</pre>
<pre>propagating/redcost/freq = 1</pre>
<pre>propagating/rootredcost/freq = 1</pre>
<pre>propagating/vbounds/freq = 1
</pre>
Kind regards,<br>
Gregor<br>
<br>
<br>
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<div>Thanks for your time (and thanks for SCIP!!!)</div>
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<div>Best regards,</div>
<div>Eleonora Pippia</div>
<br>
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