hp java heap dump analysis workshop

© Copyright 2013 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

Java Heap Dump AnalysisMadhavan Marimuthu / August 5, 2014

© Copyright 2014 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice. HP Confidential.2 © Copyright 2013 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice.

Goals of this presentation

• JVM Memory Model

• Three Main Area of Heap

• Creating Heap Dump & Options

• Useful Heap Dump Analyzing Tools

• IBM Heap Analyzer & Other Tools Demo

• Analysis Java Heap Leak Patterns

• Typical Cause of Memory leaks

• Java Coding Practice Overview

© Copyright 2014 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice. HP Confidential.3

JVM Memory Model

Thread & StackMethod invocations and the local variables are stored

State of the method including which line of code is executing

and the values of all local variables

Threads have their own call stack

Heap Snapshot of the complete Java object graph at a certain point in

time

Includes all objects, fields, primitive types and object references

Non-Heap Permanent Generation

JVM 6.0


JVM 6.0 is made up of

JVM Memory Model


Three Main Areas Impacting

Performance

Young Generation Short lived and smaller footprints

Newly Created objects

Broken down into three areas as Eden and Survivor spaces

Old Generation GC runs less frequently

Where older objects are maintained Larger Footprints

Permanent Generation Meta data about the objects in the heap

Information about classes and methods


Three Main Areas Impacting

Performance


OutOfMemoryError

In the heap,if cannot reclaim enough memory for a new object.

• Exception in thread "main": java.lang.OutOfMemoryError: Java heap space

• Exception in thread "main": java.lang.OutOfMemoryError: Requested array size exceeds VM limit

In PermGen, If there is not enough memory in the method area for creating a

new class

• Exception in thread "main": java.lang.OutOfMemoryError: PermGen space

In thread exclusive memory areas

• Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread

• Exception in thread "main": java.lang.OutOfMemoryError: <reason> <stacktrace> (Native method)

In JVM stack, too many frames on the stack

Exception in thread "main": java.lang.OutOfMemoryError: request <size> bytes for <reason>. Out of

swap space?


Memory Leak - Overview

• Objects, which can be reached via a GC root reference,

but are actually no longer used by application code.

• Too many or too large objects. Ex: Cache like structures.

• Too many temporary objects. So there is just for a short

time not enough memory. Ex: Load Scenario


Creating Heap dump

Manual kill –QUIT or kill -3

JVM parameters -XX:+HeapDumpOnOutOfMemoryError

-XX:HeapDumpPath=/directory/subdirectory

Used for production system, creates heap dump when

OutOfMemory is thrown

jmap -dump:live,format=b,file=heap.dump <pid> Part of JDK

Used to create heap dump on demand


Creating Heap dump

jmap -dump:live,format=b,file=heap.dump <pid>

Part of JDK

Used to create heap dump on demand

JVM parameters

-XX:+HeapDumpOnOutOfMemoryError -

XX:HeapDumpPath=/directory/subdirectory

Produces file java_pid<pid>.hprof

Used for production system, creates heap dump when

OutOfMemory is thrown

Useful Analyzing Tools

IBM HeapAnalyzer

IBM Pattern Modeling and Analysis Tool for Java Garbage Collector

IBM Thread and Monitor Dump Analyzer for Java


• IBM HeapAnalyzer

• IBM Thread and Monitor Dump Analyzer for Java

• Memory Analyze Tool

• VisualVM


IBM Heap Analyzer Setup

What do you need ?

• Need Standard Edition version 6 or higher Java Runtime Environment (JRE)

to run this tool

Usage <Java path>java -Xmx[heapsize] -jar ha<HeapAnalyzer version>.jar

heapdump file>

For example:

/usr/java60/bin/java -Xmx1000m -jar ha146.jar heapdump1234.txt

• 64-bit Java Runtime Environment for large Java heap dump, if more than

4GB memory is needed


Related Vocabulary

Root object :An object for which no (different) object holds a reference

Parent object :An object (for example, A) that holds at least one reference to

some(different) object (for example, B). In this case, A is said to be the parent of B

Owner object: If an object has more than one parent object, a parent object is chosen

as owner object

Child object :An object (for example, B) for which at least one (different) object(for

example, A) holds a reference. In this case B is said to be the child of A

Type: Collection of same objects


Related Vocabulary


Related Vocabulary ( Cont..)

Size: The size of an object is the amount of memory that is required to hold that

object in memory

Total Size : The sub tree size of an object is the sum of its size and the sizes of all

the objects that it reached from its children

Total size Drop: a parent and its child, it’s called a total size drop.

Total size is calculated only with owner objects.

Leak Size: Total size of leak suspects. If the leak suspects contains another leak

suspect, leak size is subtracted by nested leak suspect's total size


Related Vocabulary ( Cont..)

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Analysis Java Heap Leak Patterns

1. Horizontal memory leak: Many objects

are referenced from same objects

2. Vertical memory leak: Many objects

are linked together

3. Diagonal memory leak: Combination

of horizontal and vertical memory leak.

It's often found in tree structures


IBM HeapAnalyzer-Demo


IBM HeapAnalyzer Demo -Topics

· Parent and Children View from Table View and Tree View

· Address search

· Root List View

· Same Type View

· Calculates size of each objects

· Calculates total size of each subtree

· Finds total size drop in a subtree

· Shows gaps between objects by size

· Shows objects by size

· Shows objects by total size

· Shows objects by number of child

· Shows types by size

· Shows gap distribution chart

· Shows detailed information of an object


IBM Thread and Monitor Dump

Analyzer - Demo


IBM TMDA Demo -Topics

• Thread detail view

• Monitor detail view

• List of hang suspects

• Thread comparison view

• Thread comparison summary

• Java Monitor lock comparison view


Memory Analyze Tool- Demo


MAT Demo -Topics

• Running Leak Suspect Report

• List the Biggest Objects

• Finding Responsible Objects

• Querying Heap Objects (OQL)

• Analyze Class Loader

• Analyzing Threads

• Analyzing Java Collection Usage

• Analyzing Finalizer

• Comparing Objects


VisualVM- Profiling



IBM HeapAnalyzer

IBM Pattern Modeling and Analysis Tool for Java Garbage Collector

IBM Thread and Monitor Dump Analyzer for Java

24

Profiling

Heap profiling

When: GC runs frequently, large Java heap

Look for: very large allocated objects, small objects allocated at a

high rate

Output: throughput/responsiveness issues, memory allocation

patterns, memory strategies (alternative APIs, caching solutions)

CPU profiling

When: large amount of CPU time

Look for: methods with a high self-time

Output: throughput issues, identify algorithm and designs issues


Memory leaks: reference to allocated object remains unintentionally

reachable and cannot be GC'ed.

When: Java heap grow over time w/o bound, OOM errors

Look for: abnormal memory consumption,

Output: code errors

Profiling (Cont.)

• Lock contention: large number of context switches, related to CPU

utilization

• When: manual use of threads

• Look for: high number of context switches, many threads in wait

state

• Output: code errors. Tools for profiling

• Jprofiler

• jmap, jhat

• VisualVM


Typical Cause of Memory leaks• Programming errors, bugs

• Unchecked collections: Array,list,hashmap growth

• Not closing JDBC prepared statements

• Not closing scokets,File Handles

• Thread Leaks,handle leaks

• Class Loader Leaks

• Resource allocated outside of JVM


Collections exist in large numbers in many Java applications

Java Collections

Collection Default Capacity Empty Size 10k Overhead Expansion

HashSet 16 144 360k x2

HashMap 16 128 360k x2

Hash Table 11 104 360k X2+1

Linked List 1 48 240k +1

ArrayList 10 88 40k X1.5

String Buffer 16 72 24 x2

■ Collections that contain empty space introduce additional overhead

■ Default collection size may not be appropriate for the amount of data

being held

■ Example: IBM WebSphere Application Server running

PlantsByWebSphere

–When running a 5 user test load, and using 206MB of Java heap:

– 16% of the Java heap used just for the collection objects !!


Collections Memory foot print Order

Java Collections(Cont..)

Each Java Collection has a different level of function, and memory

overhead

Using the wrong type of collection can incur significant additional

memory overhead


• Heap and lock contention issues:

• Avoid create objects where primitives could be used (autoboxing is not

for free)

• Avoid String concatenations, use StringBuilder with append()

• Avoid StringBuilder overallocation, set initial size to reduce resizing

• Avoid use of synchronized keyword, look for a java.util.concurrent

solution

• Use java.util.concurrent API instead of "synchronized" Collections

• Don't use exceptions for flow control (Exception objects allocation), use

flow control constructs (if/else, return, switch).

• Don't generate stack traces, they're expensive operations.

• Reduce number of system calls:

• Avoid accessing disk: use large buffers or direct buffering

• Avoid accessing to underlying OS

• Avoid processing bytes and characters individually

• Never implement finalize(): A big pain for GC

Java Coding Practice High-Level

© Copyright 2014 Hewlett-Packard Development Company, L.P. The information contained herein is subject to change without notice. HP Confidential.

Download References

Visual VM 1.3.6 for Java6

https://java.net/projects/visualvm/downloads/download/release136/visualvm_136.zip

IBM Heap Analyzer

ftp://public.dhe.ibm.com/software/websphere/appserv/support/tools/HeapAnalyzer/ha4

55.jar

IBM Thread and Monitor Dump Analyzer

ftp://public.dhe.ibm.com/software/websphere/appserv/support/tools/jca/jca455.jar

Memory Analyzer Tool

http://www.eclipse.org/downloads/download.php?file=/mat/1.4/rcp/MemoryAnalyzer-

1.4.0.20140604-win32.win32.x86.zip



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