[jira] [Created] (SOLR-10506) Possible memory leak upon collection reload

["Torsten Bøgh Köster (JIRA)" <ji...@apache.org>]

Torsten Bøgh Köster created SOLR-10506:
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             Summary: 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


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 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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