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https://issues.apache.org/jira/browse/MNG-7509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17564491#comment-17564491
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Xiong Luyao edited comment on MNG-7509 at 7/9/22 3:01 AM:
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The reason is, when maven resolving the relationship, for each library it will
create instances for the dependencies in dependencyManagement. So for those
dependencies in parent pom, they will be created many times even they are
totally same. In the worst cases, If a project introduce 3000 libraries and
each library introduce the same parent pom which managed 3000 libraries
version. There will be 9 million Dependency instance created. But in fact, only
3000 Dependency instances are necessary for this case.
I think we can resolve it by adding a cache map for the dependencies. If the
dependency instance is already in the cache map, it will get instance from the
map rather than creating a new one. Fortunately, the Dependency(including
DefaultArtifact in it) is designed as immutable, which means, if someone change
the value in somewhere, it won’t change the existing instance, but return a
totally new one. So we needn’t be worried about that it will impact other
reference of a Dependency if somewhere change its value.
Here is the PR: [https://github.com/apache/maven/pull/764]
was (Author: JIRAUSER292130):
The reason is, when maven resolving the relationship, for each library it will
create instances for the dependencies in dependencyManagement. So for those
dependencies in parent pom, they will be created many times even they are
totally same. In the worst cases, If a project introduce 3000 libraries and
each library introduce the same parent pom which managed 3000 libraries
version. There will be 9 million Dependency instance created. But in fact, only
3000 Dependency instances are necessary for this case.
The way to resolve it is to add a cache map for the dependencies. If the
dependency instance is already in the cache map, it will get instance from the
map rather than creating a new one. Fortunately, the Dependency(including
DefaultArtifact in it) is designed as immutable, which means, if someone change
the value in somewhere, it won’t change the existing instance, but return a
totally new one. So we needn’t be worried about that it will impact other
reference of a Dependency if somewhere change its value.
Here is the PR: https://github.com/apache/maven/pull/764
> Huge memory cost when parent pom widely used in a big project for
> dependencyManagement
> --------------------------------------------------------------------------------------
>
> Key: MNG-7509
> URL: https://issues.apache.org/jira/browse/MNG-7509
> Project: Maven
> Issue Type: Improvement
> Components: Performance
> Reporter: Xiong Luyao
> Priority: Major
> Attachments: image-2022-07-09-09-37-53-823.png,
> image-2022-07-09-09-38-26-354.png, image-2022-07-09-10-27-12-668.png,
> image-2022-07-09-10-27-56-437.png, image-2022-07-09-10-28-05-706.png,
> image-2022-07-09-10-28-22-864.png, image-2022-07-09-10-28-35-341.png,
> image-2022-07-09-10-28-40-612.png, image-2022-07-09-10-29-04-045.png,
> image-2022-07-09-10-29-15-822.png, image-2022-07-09-10-29-21-991.png,
> image-2022-07-09-10-29-46-216.png, image-2022-07-09-10-29-51-456.png
>
>
> When maven try to resolve dependency relationship, it will create many
> instances of dependency / artifact, even the dependency/artifact content is
> totally same, but just in different pom models. It cost huge memory if there
> is a parent pom with dependencyManagement which managed a lot of
> dependencies, and this parent pom are implemented by many project libraries.
> (libraries_count * managedDependency_count) dependency instances will be
> created. For example, if there are 3000 libraries, and all the library
> introduce same parent pom which managed 3000 dependencies version. There will
> be 3000*3000 =9,000,000 dependency instances will be created. But most of
> them are same, in fact, we only need one instance for each dependency in
> parent pom (3000 dependency instances).
>
> Here is a real case in enterprise level project. We have about 3000 business
> domain libraries, there are dependency relationship between them. We need to
> build all libraries in one release to keep all the libraries in same release
> are based on same code. So we used a parent pom to manage all the version for
> a release, and introduced by those libraries. As below picture, when the
> release start, it will calculate and start with the library which doesn't
> depend on others, then start the library which dependency libraries are
> already built. Keep this process until all libraries are built.
> With current maven resolve logic, it costs huge memory in above ways to built
> libraries. And even the libraries have been released, if the project which
> contains a lot of above libraries, it also cost huge memory when building
> project.
>
> !image-2022-07-09-09-37-53-823.png|width=493,height=226!
>
> Here is a thread dump when building a real project which depends on about
> 1000 above libraries. The top 5 objects are all related to
> org.eclipse.aether.graph.Dependency.
> !image-2022-07-09-09-38-26-354.png|width=510,height=199!
>
>
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