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Re: Meaning of "score" when looking for package solutions



On Fri, May 22, 2009 at 10:14:24AM -0500, "Boyd Stephen Smith Jr." <bss@iguanasuicide.net> was heard to say:
> In <886fa7aa0905220106n791648e1v6b79acecc06b34c9@mail.gmail.com>, Jason 
> Filippou wrote:
> >Being new to Debian and running Squeeze I'm curious as to what the "Score
> > is x" line's supposed to mean when examining solutions to solve package
> > dependences through aptitude.
> 
> Aptitude assigns each possible resolution a score.  A resolution's score is 
> the sum of the scores of all actions in the resolution.  Each possible 
> action has a fixed, integral score.  The defaults for these scores and the 
> configuration option to alter them are documented 
> http://algebraicthunk.net/~dburrows/projects/aptitude/doc/en/ch02s04s05.html 
> and in the package(s) aptitude-doc-$lang.
> 
> Those documents contain all the apt.conf settings that will affect aptitude, 
> the ones that affect action score are all of the form:
> Aptitude::ProblemResolver::${Action}Score.
> 
> When presenting the user with a solution, aptitude will show the one with 
> the highest score that the resolver has found so far.  Solutions that are 
> found to have a score that is too low will be immediately rejected.

  This is basically correct (as you noted earlier, higher scores are
better).  I want to highlight a subtle point: you get the highest
scored solution *so far*.  aptitude sometimes skips over good solutions
because it has to do something that looks bad (like upgrading gnome and
breaking lots of dependencies in the process) in order to get there.

  The score is something of a "blinkenlights" feature, meaning that
it's fun to see for people who are strange like me, but it doesn't
really tell you anything useful about the solution.  It's just a window
into aptitude's head.  Future releases display the solution's tier
instead (a new feature), which I hope is a bit more useful.

  Daniel


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