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https://issues.apache.org/jira/browse/SOLR-10506?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16004822#comment-16004822
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Mike Drob commented on SOLR-10506:
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Well, the stopWatching method is only called when a core is closed. Maybe it
should be called in postClose to be safe? I'm not familiar enough with the
internals of a SolrCore lifecycle to judge this.
The implementation details of it being a weak reference mean that it can return
null without stopWatching/clear ever being called. If the GC decides that it
needs memory then your schema reader will suddenly be gone. So an open core
using lots of memory will eventually lose the reference before it should.
> Possible memory leak upon collection reload
> -------------------------------------------
>
> Key: SOLR-10506
> URL: https://issues.apache.org/jira/browse/SOLR-10506
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: Server
> Affects Versions: 6.5
> Reporter: Torsten Bøgh Köster
> Attachments: solr_collection_reload_13_cores.png,
> solr_gc_path_via_zk_WatchManager.png
>
>
> Upon manual Solr Collection reloading, references to the closed {{SolrCore}}
> are not fully removed by the garbage collector as a strong reference to the
> {{ZkIndexSchemaReader}} is held in a ZooKeeper {{Watcher}} that watches for
> schema changes.
> In our case, this leads to a massive memory leak as managed resources are
> still referenced by the closed {{SolrCore}}. Our Solr cloud environment
> utilizes rather large managed resources (synonyms, stopwords). To reproduce,
> we fired out environment up and reloaded the collection 13 times. As a result
> we fully exhausted our heap. A closer look with the Yourkit profiler revealed
> 13 {{SolrCore}} instances, still holding strong references to the garbage
> collection root (see screenshot 1).
> Each {{SolrCore}} instance holds a single path with strong references to the
> gc root via a `Watcher` in `ZkIndexSchemaReader` (see screenshot 2). The
> {{ZkIndexSchemaReader}} registers a close hook in the {{SolrCore}} but the
> Zookeeper is not removed upon core close.
> We supplied a Github Pull Request
> (https://github.com/apache/lucene-solr/pull/197) that extracts the zookeeper
> `Watcher` as a static inner class. To eliminate the memory leak, the schema
> reader is held inside a `WeakReference` and the reference is explicitly
> removed on core close.
> Initially I wanted to supply a test case but unfortunately did not find a
> good starting point ...
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