This document summarizes a presentation on practical lessons for memory analysis. It discusses analyzing memory leaks, heavy requests, and reducing memory footprint. It also covers non-heap memory issues like class loader leaks. The presentation demonstrates analyzing heap dumps with Memory Analyzer to find the largest objects, track object retention, find redundant objects, and analyze class loader leaks. The goal is to provide tips for solving common memory problems using heap analysis tools.

1. Practical Lessons in
Memory Analysis
Andrew Johnson
advisory software engineer
IBM United Kingdom Limited

2. 2
Based on TS-4118 at JavaOne 2009
Original presentation by
• Krum Tsvetkov - SAP AG
• Andrew Johnson – IBM United Kingdom Limited

3. 3
Goal
Learn practical tips and tricks for the
analysis of common memory-related
problems

4. 4
Agenda
Introduction
Analyzing Memory Leaks
Analyzing a Heavy Request
Reducing Memory Footprint
Non-heap Memory
Summary
Q & A

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Why care about memory?
Memory is a crucial resource
Excessive memory usage can cause:
• Crashes in the application
• Unresponsive applications
• Unpredictable program behavior
Memory analysis is complicated

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How will this session help me?
Present common memory-related issues
Give hints how to solve them using heap
dumps and Memory Analyzer
Show many demonstrations using real-life
heap dumps
Show how to simplify analysis by
automating it

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Heap dumps are useful for analysis
Heap dumps come “for free”
They are suitable for production
The comprehensive data allows detailed
analysis
There is wide platform coverage
• HPROF dumps from HotSpot based JVMs
• DTFJ system dumps from IBM JVMs
• Portable Heap Dumps (PHD) file from IBM JVMs
JSR-326/Apache Kato standardization
• http://incubator.apache.org/projects/kato.html

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Agenda
Introduction
Analyzing Memory Leaks
Analyzing a Heavy Request
Reducing Memory Footprint
Non-heap Memory
Summary
Q & A

9. 9
First, collect data for analysis
Enable heap dumps on OutOfMemoryError
Trigger heap dumps on demand if needed
How-to for getting a heap dump:
http://wiki.eclipse.org/index.php/MemoryAnalyzer#Gett

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How to get a “good” heap dump
 When memory is exhausted the leak will occupy
most of the processing space
 Ensure big enough processing space, this will make
the leak easier to find
Max
Heap
Min
Heap
Processing
Space
Base
Memory

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How to analyze memory leaks
Find the biggest objects
Analyze why they are kept in memory
Analyze what makes them big

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Demonstration
 Analysis of a Memory Leak

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How the leak analysis was done
Got a “good” heap dump
Found the biggest objects (in the dominator
tree)
Analyzed who kept them alive (using paths)
Analyzed what made them big (looking at
their retained set)
Used “Leak Report” to automate the
analysis

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Agenda
Introduction
Analyzing Memory Leaks
Analyzing a Heavy Request
Reducing Memory Footprint
Non-heap Memory
Summary
Q & A

15. 15
What is a heavy request?
An expensive operation which can cause an
OutOfMemoryError
“Acute” rather than a “chronic” problem
Examples:
• Reading a big file in memory not in chunks
• Try to load a whole DB table in memory

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Inspect the thread to find its activity
Look at the thread attributes - name, class,
etc...
Look at the local variables
Look at the stack of the thread

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Demonstration
Analysis of a Heavy Request

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How thread activity was analyzed
Inspected thread attributes and local
variables (in object explorer)
Analyzed stack trace

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Agenda
Introduction
Analyzing Memory Leaks
Analyzing a Heavy Request
Reducing Memory Footprint
Non-heap Memory
Summary
Q & A

20. 20
Why is memory footprint important?
 A lower memory footprint can improve:
• The scaling of an application / server
• Performance by increasing the time between GCs
Max
Heap
Min
Heap
Processing
Space
Base
Memory

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Find where footprint can be optimized
Inspect the set of retained objects
Search for inefficiently used data structures
Look for redundant data

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Demonstration
• Analysis of High Memory Footprint

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How memory footprint was analyzed
Analyzed retained objects (in dominator
tree, retained set)
Used “Group by Value” to find redundant
objects
Used the commands from the “Collections”
group
Used “Component Report” to automate the
analysis

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Agenda
Introduction
Analyzing Memory Leaks
Analyzing a Heavy Request
Reducing Memory Footprint
Non-heap Memory
Summary
Q & A

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Non-heap memory can be exhausted
What is in the non-heap memory?
• Metadata for classes
• Interned Strings (for some JVMs)
• NIO direct byte buffers
• Bitmaps
How can it be exhausted?
• Too many interned Strings
• Classes packaged and loaded multiple times
• Class loaders which are not properly released

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Why is a class loader not released?
 The trivial case – there
is a “normal” reference
to it
 A common case – it is
the context class loader
of a thread
 The registry problem –
one instance is enough
Class loader
Class Class
Instance Instance Instance InstanceInstance
Registry
Root

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How to analyze “leaking” class loaders
Find classes loaded more than once
Find redundant loaders by their name
Look at the paths to the suspect class
loaders

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Demonstration
Analysis of “Leaking” Class Loaders

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How “leaking” class loaders analysis was done
Used the “Duplicate Classes” query
Found the loaders that load redundant
classes
Analyzed why they are not released (using
paths)

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Agenda
Introduction
Analyzing Memory Leaks
Analyzing a Heavy Request
Reducing Memory Footprint
Non-heap Memory
Summary
Q & A

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Summary
Heap dumps are very helpful for memory
analysis
Memory Analyzer offers rich analysis feature
set
The analysis can often be automated

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Questions and answers
Memory Analyzer homepage @ Eclipse
• http://www.eclipse.org/mat/
• Binary downloads
• Source downloads
• Forum
• Getting started guide
• Frequently asked questions
• Bug reports

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Thank you
Andrew Johnson
• IBM United Kingdom Limited
Memory Analyzer homepage @ Eclipse
• http://www.eclipse.org/mat/
IBM Support Assistant
• http://www-01.ibm.com/software/support/isa/
• IBM Monitoring and Diagnostic Tools – Memory
Analyzer