This document discusses analyzing Java heap dumps to diagnose out of memory errors. It begins with an overview of Java heap concepts like how memory is allocated and garbage collection. Next, it defines what a heap dump is and how to generate one. It then explains how to analyze heap dumps using tools like MAT to identify the largest objects consuming memory, visualize object reference graphs and dominator trees, and investigate threads. The goal is to find memory leaks and reduce memory usage to prevent out of memory errors from occurring.

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AEM | Heap Dump Analysis
Kanika Gera

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Heap Dump Analysis
• Java Heap Size & Concepts
• Concepts of Heap Dumps
• Memory Analyzer tools
• MAT
• Demo

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Java Heap Size & Concepts
 When a Java program starts, Java Virtual Machine gets some
memory from Operating System. Java Virtual Machine or JVM
uses this memory for all its need and part of this memory is call
java heap memory.
 The flag Xmx specifies the maximum memory allocation pool for
a Java Virtual Machine (JVM), while Xms specifies the initial
memory allocation pool. This means that your JVM will be started
with Xms amount of memory and will be able to use a maximum
of Xmx amount of memory.
 JVM also run garbage collector periodically to reclaim memory
back from dead objects. JVM expands Heap in Java some where
near to Maximum Heap Size specified by -Xmx and if there is no
more memory left for creating new object in java heap , JVM
throws java.lang.OutOfMemoryError and your application/AEM
dies by throwing OOM.
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What is a HeapDump?
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• A snapshot of the memory of a Java process
• It contains information about the java objects and classes in the heap:
All Objects - Class, fields, primitive values and references
All Classes - Classloader, name, super class, static fields
Garbage Collection Roots - Objects defined to be reachable by the JVM
Thread Stacks and Local Variables - The call-stacks of threads at the moment of the snapshot, and per
frame information about local objects
To generate heap dump we have to execute a JVM with the following parameters in eclipse
-XX:+HeapDumpOnOutOfMemoryError writes heap dump on first OutOfMemoryError
Here the heap dump will be generated in the “current directory” of the JVM by default. It can be explicitly
redirected with
-XX:HeapDumpPath= for example -XX: HeapDumpPath=/disk/myFolder.

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Shallow, Retained Heap
 Shallow heap is the memory consumed by one object. An object needs 32 or 64 bits
(depending on the OS architecture) per reference, 4 bytes per Integer, 8 bytes per Long,
etc. Depending on the heap dump format the size may be adjusted (e.g. aligned to 8,
etc...) to model better the real consumption of the VM.
 Retained set of X is the set of objects which would be removed by GC when X is garbage
collected.
 Retained heap of X is the sum of shallow sizes of all objects in the retained set of X, i.e.
memory kept alive by X.

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Shallow vs Retained Heap
 The retained set for a leading set of objects, such as all objects of a particular class or all
objects of all classes loaded by a particular class loader or simply a bunch of arbitrary
objects, is the set of objects that is released if all objects of that leading set become
unaccessible. The retained set includes these objects as well as all other objects only
accessible through these objects. The retained size is the total heap size of all objects
contained in the retained set.

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Object Reference Graph

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Dominator Tree
 An object x dominates an object y if every path in the object graph from the start (or the root) node to y
must go through x.
 The immediate dominator x of some object y is the dominator closest to the object y.
 A dominator tree is built out of the object graph. In the dominator tree each object is the immediate
dominator of its children, so dependencies between the objects are easily identified.
The dominator tree has the following important properties:
 The objects belonging to the sub-tree of x (i.e. the objects dominated by x ) represent the retained set of
x .
 If x is the immediate dominator of y , then the immediate dominator of x also dominates y , and so on.
 The edges in the dominator tree do not directly correspond to object references from the object graph.

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Object graph to Dominator Tree

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Heap Analysis using MAT

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Heap Dump – Biggest Objects
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Heap Dump – Histogram
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Heap Dump – Histogram
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Analyzing Threads
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