Yougoss commented on PR #768:
URL: https://github.com/apache/maven/pull/768#issuecomment-1208283097
> The idea of maintaining pool of immutable objects is good, however the
implementation proposed isn't: the JVM is free to cleanup dependencyCache as
soon as execution leaves #toDependency method - there are no more references to
depCacheKeykey. The author is observing some performance improvements just
because his JVM has enough free memory to not trigger GC whilst maven builds
dependency graph (give JVM more memory to save memory).
Hi @andreybpanfilov, glad to see your comments, I think there may be some
misunderstanding for this change.
So first, I need to declare that the main change I want to make should be in
ArtifactDescriptorReaderDelegate(maven-resolver-provider), but I noticed that
there is same code in RepositoryUtils(maven-core), maybe for some historic
reason, so I just made a same change here to keep them consistent.
For the toDependency method in RepositoryUtils only collect
dependencyManagement in project pom, as you side, the change here doesn't bring
too much increasement.(Entrance: getDependencies:243,
LifecycleDependencyResolver (org.apache.maven.lifecycle.internal))
But for the convert method in ArtifactDescriptorReaderDelegate is totally
different. It's used for calculating all the dependency relationship. It will
travels all the dependencies in the project pom, and collect dependency
manageMent for each dependency.
Image one case, there are two dependencies in the project. And in each
dependency pom, there are two dependencyManagement, so there should be 4
dependency instance will be created to save the information of the
dependencyManagement.
Image a case which is a little complicated, if the dependenyManagement in
above two dependencise are one same parent pom, and there are 1000 dependencies
managed in this parent pom. There would be 1000 * 2 = 2000 instance will be
created for dependencyManagement, although there is only 1000 after remove
duplicate ones.
So for our real case in ebay project, there is about 3000 dependencies with
a parent pom which managed all these 3000 dependencies version. So when
building the project, there will be 3000 * 3000 = 9,000,000 instance will be
created for dependencyManagement. But after removing duplicate, there are only
3000 are necessary. That's the change here why I use a hashmap the cache all
the dependencyManagement instance.
And why I use a weakHashMap here is I want to make sure when the dependency
instance are not refered by graph anymore it can be cleanup by GC, won't stay
in memory since the reference from hashMap key.
And for the Memory cost you mentioned, I have also shared a build comparison
for our project with command 'export MAVEN_OPTS="-Xms1g -Xmx1g”' to limit the
memory in jira ticket. With the PR, it can build successfully, but with
current maven code, it will throw OOM exception.
Please let me know if I should share some fake project to simulate this case
to make you more clear
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