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POFinding Java memory leak when not all used heap is reachable from threads
 

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    <p>I am looking into a potential memory leak (or at least memory waste) in a largish Java based system. The JVM is running with a maximum heap size of 5 GB and 2-3GB heap usage is an expected base line for the application. (There can be peaks that are higher)</p> <p>In an overload scenario which I am investigating the heap gets filled up. Analyzing the a heap-dump with the "Eclipse MemoryAnalyzer Tool" shows (no surprise) that the heap is entirely used up.</p> <p>MAT shows 2 potential leak candidates, both roughly retaining 2.5GB: java.lang.Thread and a domain object from the system which is used extensively during transaction processing in the system. All these domain objects are however (no surprise) reachable from the Thread instances. Those threads are processing the transactions, after all. Thus, the 2.5 GB attributed to java.lang.Thread is almost entirely caused by those domain objects. No surprise here.</p> <p>Listing the object tree of all java.lang.Thread instances and summing up the retained heap of all threads results in 2.5 GB of retained heap.</p> <p>Where should I look for the other 2.5 GB that are needed to fill up the heap, if they are not reachable from an instance of java.lang.Thread? - There is nothing in the finalizer queue - There is not a significant amount of unreachable objects pending GC</p> <p>I think another way to put this question is: "How do I find all objects that are not reachable from an instance of java.lang.Thread? Maybe an OQL query?, and the other question: "What kind of Objects are there that are not reachable from an instance of java.lang.Thread other then Objects in the Finalizer Queue and unreferenced objects pending GC?"</p>
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    1. COWould you mind providing some more data. Like maybe the graph you are speaking of and the Domain objects themselves. How they are used. Do they make a large use of oddly sized arrays? Is there an allocation which fails and causes an OOM to be thrown?
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      1. USGreg Giacovelli
    2. COPlease also include whether or not you are using GC flags (like: -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled -XX:+UseConcMarkSweepGC)
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    3. COAre you sure the 2.5GB from the thread is also including the domain objects? If they are two separate counts then that is going to total your 5GB.
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      1. USTim B
 

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