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From "Richard S. Hall (JIRA)" <j...@apache.org>
Subject [jira] Updated: (FELIX-2528) Potential performance issue in resolver when uses constraint conflict is detected
Date Fri, 06 Aug 2010 16:09:15 GMT

     [ https://issues.apache.org/jira/browse/FELIX-2528?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Richard S. Hall updated FELIX-2528:
-----------------------------------

    Description: 
Whenever the resolver detects a uses constraint conflict, it tries to create two permutations
of its solution search space. Since a conflict effectively arises between two parties (an
existing package constraint and a package constraint being added), the algorithm creates a
potential solution permutation removing the opposite party from each, since it doesn't know
which one may ultimately lead to a correct solution. The permutation removing the added package
constraint candidates is called a "uses" permutation (since it permutates the package being
used) while the permutation removing the existing package constraint is called an "import"
permutation (since it generally is causing a backtrack on a previously selected imported package).

The algorithm is basically depth-first search, which ultimately results in it giving priority
to the "uses" permutations. This means it won't backtrack on any choices for imports until
it determines that a previous choice was incorrect. This appears to work fairly well in practice.
The downside is that it is possible that we keep detecting conflicts related to an incorrect
import decision before determining that the original import decision was incorrect. This results
in lots of import permutations being generated that are effectively smaller and smaller subsets
of each other. This can consume a lot of memory which slows things down as well as creates
lots of largely repetitive permutations to process.

In short, we need to try to detect if we've already permutated an import and not do it again.


  was:
Whenever the resolver detects a uses constraint conflict, it tries to create two permutations
of its solution search space. Since a conflict effectively arises between two parties (an
existing package constraint and a package constraint being added), the algorithm creates a
potential solution permutation removing the opposite party from each, since it doesn't know
which one may ultimately lead to a correct solution. The permutation removing the added package
constraint candidates is called a "uses" permutation (since it permutates the package being
used) while the permutation removing the existing package constraint is called an "import"
permutation (since it generally is causing a backtrack on a previously selected imported package).

The algorithm is basically depth-first search, which ultimately results in it given priority
to the "uses" permutations. This means it won't backtrack on any choices for imports until
it determines that a previous choice was incorrect. This appears to work fairly well in practice.
The downside is that it is possible that we keep detecting conflicts related to an incorrect
import decision before determine that the original import decision was incorrect. This results
in lots of import permutations being generated that are effectively smaller and smaller subsets
of each other. This can consume a lot of memory which slows things down as well as creating
lots of largely repetitive permutations to process.

In short, we need to try to detect if we've already permutated an import and not do it again.



> Potential performance issue in resolver when uses constraint conflict is detected
> ---------------------------------------------------------------------------------
>
>                 Key: FELIX-2528
>                 URL: https://issues.apache.org/jira/browse/FELIX-2528
>             Project: Felix
>          Issue Type: Improvement
>          Components: Framework
>    Affects Versions: framework-3.0.0, framework-3.0.1
>            Reporter: Richard S. Hall
>            Assignee: Richard S. Hall
>             Fix For: framework-3.2.0
>
>
> Whenever the resolver detects a uses constraint conflict, it tries to create two permutations
of its solution search space. Since a conflict effectively arises between two parties (an
existing package constraint and a package constraint being added), the algorithm creates a
potential solution permutation removing the opposite party from each, since it doesn't know
which one may ultimately lead to a correct solution. The permutation removing the added package
constraint candidates is called a "uses" permutation (since it permutates the package being
used) while the permutation removing the existing package constraint is called an "import"
permutation (since it generally is causing a backtrack on a previously selected imported package).
> The algorithm is basically depth-first search, which ultimately results in it giving
priority to the "uses" permutations. This means it won't backtrack on any choices for imports
until it determines that a previous choice was incorrect. This appears to work fairly well
in practice. The downside is that it is possible that we keep detecting conflicts related
to an incorrect import decision before determining that the original import decision was incorrect.
This results in lots of import permutations being generated that are effectively smaller and
smaller subsets of each other. This can consume a lot of memory which slows things down as
well as creates lots of largely repetitive permutations to process.
> In short, we need to try to detect if we've already permutated an import and not do it
again. 

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