Overview
Identifying reason for the out of memory error of java
applications with larger heap size may be one of the nightmares of a developer,
because most of the out of memory situations may not be identified during the
testing phase. It may occur only in production after running for a long time.
The purpose of this article is to explain the use of Heap analyser tool to
identify the memory leakage of larger enterprise java applications, which uses
larger size of heaps. Before going to the details we will discuss about
following points.
- Heap
- Garbage Collection
- Out of memory
What is heap?
The space used by the Java Runtime to allocate memory to
Objects and JRE Classes is called Heap. The heap space can be configured using
the following JVM arguments.
o
-Xmx<size> - Setting maximum Java heap
size
o
-Xms<size> - Setting initial Java heap
size
The maximum heap size can be configured in a 32 bit JVM is
2GB. If any application requires more than 2 GB, it should run on 64 bit JVM.
64MB is the maximum heap size by default.
Garbage Collection
One of the advantages of Java over C++ is automatic memory
management. In C++ memory can be released manually, but it will happen
automatically in Java using a process call Garbage collection. Garbage
collection will free up the memory of the object that doesn’t have any
reference. That is will destroy the unused objects. Garbage collection process
can be tune for different applications based on the object creation
characteristics of the application. This can be achieved through number of JVM
arguments. Following are the few JVM arguments which can be used to tune the
garbage collection process.
|
Option
|
Description
|
|
-XX:-UseParallelGC
|
Use parallel garbage collection for scavenges.
|
|
-XX:-UseParallelOldGC
|
Use parallel garbage collection for the full collections. Enabling
this option automatically sets -XX:+UseParallelGC.
|
|
-XX:NewRatio
|
Ratio of old/new generation sizes. The default value is 2.
|
|
-XX:SurvivorRatio
|
Ratio of eden/survivor space size. The default value is 8.
|
|
-XX:ParallelGCThreads
|
Sets the number of threads used during parallel phases of the garbage
collectors. The default value varies with the platform on which the JVM is
running
|
GC Execution has direct relationship with the size of the
Heap.
- Larger Heap size will increases the GC execution time but decreases the number of GC executions.
- Smaller Heap Size will increases the number of GC executions but decreases the GC execution time
Out of memory
The java.lang.OutOfMemoryError
will occur when the application tries to add more objects into the heap and
there is no space left. This will happen when the maximum heap size set in the
start of application is filled with the objects and garbage collector is not
able free up the memory because the all objects in heap still have some
references. This may happen because of two reasons.
- The application may need more memory to run; because of the currently allocated heap size is not enough to accommodate the objects generated during the run time.
- The other reason may be due to the coding error the application is keeping the references of unwanted objects
The solution for the first reason is, increase the heap
size. But the solution for the second is analyzing the code flow and heap dump
to identify the unwanted objects in heap. To analyse the application heap we
need to take the heap dump and open the same in memory analyzing tool.
In this article we will discuss on how to take heap dump of
an application running on Oracle Java
and analyse the same in Eclipse Memory
Analyzer.
How to take Heap Dump
Heap dump can be take in two ways,
- JVM argument can be added to generate heap dump whenever an OutOfMemoryError occurs
-XX:+HeapDumpOnOutOfMemoryError
option can be added to generate a heap dump on OutOfMemoryError. By default the
heap dump is created in a file called java_pid pid .hprof in the working
directory of the VM. But we can set alternative path using the JVM option
-XX:HeapDumpPath=path
- Using jmap tool available with JDK. Following command can be executed from the command line.
<pid> can be replaced with process id
of the application
Eclipse Memory Analyzer
Eclipse Memory Analyzer can download from the following
location.
Unzip the downloaded file and double click ‘MemoryAnalyzer’
to start the application.
Execute a java program which is continuously running.
Take heap dump using jmap
jmap -dump:format=b,file=heap.bin 6920
Identify the process id of running application in Windows from
Task Manager
In Linux to identify the process id use ‘ps –ef | grep java’
Open the heap dump in Eclipse Memory Analyzer using the
option File Ã
Open Heap Dump
First it will prompt to create a leak suspect report. User
can create or skip the same.
In overview tab of the memory analyser will show the total
size of the heap and it will show a pie chart of object wise size.
Click on the highest value in the pie chart and select List Objects Ã
with outgoing references
It will open a new tab ‘List
Objects’ and expand the tree structure of the object.
User can expand and find the objects available in heap. In
this current example the HeapLoader object consist of a list of School object
and school object consist of a list of students.
School List in HeapLoader
Each school object in List and its attributes
Student object in school and its attribute
There is an option available in Eclipse Memory Analyser to
Acquire Heap Dump, if the application is running on same machine. Select
File à Acquire Heap Dump option will show the all Java applications available
in machine. Select the process, browse the path where the heap want to save and
click finish button.
These are the basics steps to analyse a heap dump. There are
more options available in Memory Analyser, which will give more insights into
the heap dump. And Eclipse Heap Analyser can be used for the dumps generated
from the application running on Oracle Java.
To analyse the heap dump running on IBM java can be downloaded from
Sample application
The sample application used in the above example consists of
two objects School and Student.
School will have a list of Students. The application has a
list in the main class which will load schools with students. And the
application will continue in an infinite loop. The number of schools and
students can be given as input from command line
Code is available at:
Download and import the project in Eclipse. To run the application,
export the jar from project.
Sample command to run application: java -jar hear-loader.jar 100 100
Thnq For Sharing For Good Info Vasth
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