[Scip] explanation of use of primal heuristics

[Gregor Hendel]

Hi Victor,

your last sentence stresses the fact that our testing has not been that
bad :)

The order in which heuristics are called is not determined by their
priority alone, but also by their heuristic timing (before/after a node,
before presolve, to mention only a few). Some heuristics as, e.g.,
feasibility pump, have a threshold on the number of already found
solutions and will not be run if this threshold is exceeded.
The execution of SCIP heuristics can have side effects on the overall
search-performance of the solver due to the variable statistics
collected, which is why even a fruitless call of a heuristic can still
speed up the solution process.

If you are interested in disabling all heuristics at once, you can
simply type

set heuristics emphasis off

in the SCIP interactive shell.

Best regards
gregor

Am 19.02.2013 18:57, schrieb Victor Miller:
> I have a number of IP problems which I'm using SCIP to solve.  I've
> noticed that in these problems that the various diving heuristics
> *never* find a solution.  So I started experimenting with disabling
> them, and the running time went up!  I have one small case which takes
> Scip 2.97 seconds to solve on my workstation.  I noticed that
> coefdiving took 0.11 seconds but found no solutions, so I disabled
> it.  Much to my surprise that caused the solution time to double!  By
> disabling it, it allowed other fruitless heuristics to run such as
> pscostdiving. But when I disabled those, others now came into play. 
> I'm not completely clear on how heuristics are chosen to be run.  I
> would think that it's first by priority, but what happens if a
> heuristic reports that it didn't find a solution?  So far, it's been
> like whack-a-mole.  I disable a bunch of heuristics which don't find
> solutions, and another new one pops up.  So far, not disabling any
> heuristics gives the lowest run time.
>
>
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