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<p style="margin-top:0; margin-bottom:0">Hello SCIP team,</p>
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<p style="margin-top:0; margin-bottom:0">I have a CIP model where I've noticed a big performance difference when I swap the order in which I create/add variables and constraints to the SCIP model. I first noticed this using my C-API code but have now reproduced
it using the interactive shell. </p>
<p style="margin-top:0; margin-bottom:0"><span style="font-size:12pt"><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px"><br>
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<p style="margin-top:0; margin-bottom:0"><span style="font-size:12pt"><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">This
is a job scheduling app where I have four types of jobs SAB, PPA, PWD and EXP. Performance varies greatly depending on the order in which I create the variables and constraints for these jobs. </span></span></p>
<p style="margin-top:0; margin-bottom:0"><span style="font-size:12pt"><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px"><br>
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<li><span style="font-size:12pt"><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">When I create the PWD-related variables
and constraints before the EXP variables, then it finds an optimal solution in 0.8 seconds and converges to prove optimality after 20 seconds. <br>
<br>
</span></span></li><li>But when<span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px"> I create the EXP</span><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">-related
variables and constraints before </span><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">the PWD variables, then it
takes 651 seconds to find the optimal solution and takes over 100 seconds to find the first feasible solution. <br>
<br>
</span></li><li><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">In both cases, the optimal solution is exactly the same. <br>
<br>
</span></li><li><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">In a different case, when I create SAB and PPA model before PWD
and EXP, then it also takes about 100 secs to find the optimal solution and 525 seconds to converge and prove optimality. <br>
</span></li></ul>
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<span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">I've attached two CIP input files</span><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px"> </span><span style="font-family:Calibri,Helvetica,sans-serif,Helvetica,EmojiFont,"Apple Color Emoji","Segoe UI Emoji",NotoColorEmoji,"Segoe UI Symbol","Android Emoji",EmojiSymbols; font-size:16px">where
the only difference is the sequence of the variables and constraints. The </span>variables and constraints are the same but in a different order. </div>
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Also attached the SCIP output logs from running with each of the two attached CIP in the interactive shell.
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<p style="margin-top:0; margin-bottom:0">Attached files:</p>
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<li>pwdFirst.cip = PWD vars and constraints created first (this is the fast case)</li><li>expFirst.cip = EXP vars and constraints created first (this is the slow case)</li></ul>
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<li>pwdFirstSol.rtf = SCIP output for the PWD first case (fast result)<br>
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<li>expFirstSol.rtf = SCIP output for the EXP first case (slow result)</li></ul>
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<p style="margin-top:0; margin-bottom:0">Can you please explain what is causing this order dependency, and how I might be able to eliminate or control it via SCIP parameters? For example, can I use Variable Branching options to override these ordering effects?
I'd rather the performance not depend on the order of my C++ code statements.</p>
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<p style="margin-top:0; margin-bottom:0">Thank you,</p>
<p style="margin-top:0; margin-bottom:0"> Rich Levinson</p>
<p style="margin-top:0; margin-bottom:0"> </p>
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