JVM Basic Idea

JVM(Java Virtual Machine)

JVM Architecture: JVM is the heart of any Java based Application Server. We face most of the issues due to incorrect JVM tuning. It is very important to understand the Overall architecture of the JVM in order to trouble shoot different JVM tuning related issues. Here we are going to discuss the Architecture and the Major parts of a Java Process And the Java Heap Division.

The Following Diagram is just a basic overview of a Java Process in a 2 GB process Size Machine. Usually in 32 bit Windows Operating Systems the default process size will be 2 GB (In Unix based 64 bit operating Systems it can be 4GB or more). So i draw the following Diagram of Java Process to explain the Java Process partitions in a 2Gb process size machine.

Java Process Architecture Diagram

In the above diagram we will find different partitions of a Java Process. Please compare the above diagram with below descriptions.

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1) Just for Example we can see that Process Size is 2048 MB (2GB)

2) The Java Heap Size is 1024MB (means 1GB)   -Xmx1024m

3) Native Space = ( ProcessSize – MaxHeapSize – MaxPermSize) It means around 768 MB of Native Space.

4) MaxPermSpace is around -XX:MaxPermSize=256m

5) Young Generation Space is around    40% of Maximum Java Heap.

Different parts of JVM:

1) Eden space: Eden Space is a Part of Java Heap where the JVM initially creates any objects, where most objects die and quickly are cleaned up by the minor Garbage Collectors (Note: Full Garbage Collection is different from Minor Garbage Collection). Usually any new objects created inside a Java Method go into Eden space and the objects space is reclaimed once the method execution completes. Where as the Instance Variables of a Class usually lives longer until the Object based on that class gets destroyed. When Eden fills up it causes a minor collection, in which some surviving objects are moved to an older generation.

2) Survivor Spaces: Eden Space has two Survivor spaces. One survivor space is empty at any given time. These Survivor Spaces serves as the destination of the next copying collection of any living objects in Eden and the other survivor space.

The parameter SurvivorRatio can be used to tune the size of the survivor spaces.

-XX:SurvivorRatio=6 sets the ratio between each survivor space and Eden to be 1:6

If survivor spaces are too small copying collection overflows directly into the tenured generation.

3) Young Generation: (-XX:MaxNewSize): Till JDK1.3 and 1.4 we used to set the Young Generation Size using -XX:MaxNewSize. But from JDK1.4 onwards we set the YoungGeneration size using (-Xmn) JVM option.

Young Generation size is controlled by NewRatio.  It means setting -XX:NewRatio=3 means that the ratio between the Old Generation and the Young Generation is  1:3. Similarly -XX:NewRatio=8 means that 8:1 ratio of tenured and young generation.

NewRatio: NewRatio is actually the ratio between the (YoungGenaration/Old Generations) has default values of 2 on Sparc , 12 on client Intel, and 8 everywhere else.

NOTE: After JDK 1.4 The Young Generation Size can be set using  (-Xmn) as well.

1) Virtual space-1:(MaxNewSize – NewSize): The First Virtual Space is actually shows the difference between the -XX:NewSize and -XX:MaxNewSize.  Or we can say that it is basically a difference between the Initial Young Size and the Maximum Young Size.

JavaHeapArea:( -Xmx and –Xms): Java Heap is a Memory area inside the Java Process which holds the java objects.  Java Heap is a combination of Young Generation Heap and Old Generation Heap. We can set the Initial Java Heap Size using -Xms JVM parameter similarly if we want to set the Maximum Heap Size then we can use -Xmx JVM parameter to define it.

Example:

-Xmx1024m —> Means Setting the Maximum limit of Heap as 1 GB

-Xms512m —> Means setting Java Heap Initial Size as 512m

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NOTE-1): It is always recommended to set the Initial and the Maximum Heap size values as same for better performance.

NOTE-2): The Theoretical limitation of Maximum Heap size for a 32 bit JVM is upto 4GB. Because of the Memory Fragmentation, Kernel Space Addressing, Swap memory usages and the Virtual Machine Overheads are some factors JVM does not allow us to allocate whole 4GB memory for Heap in a 32 bit JVM. So usually on 32-bit Windows Operating Systems the Maximum can be from 1.4 GB to 1.6 GB.

If we want a larger memory allocation according to our application requirement then we must choose the 64-bit operating systems with 64 bit JVM. 64-bit JVM provides us a larger address space. So we can have much larger Java Heap  with  the increased number of Threads allocation area. Based on the Nature of your Operating system in a 64 bit JVM you can even set the Maximum Heap size upto 32GB.

Example:        -Xms32g -Xmx32g -Xmn4g

2) Virtual Space-2: (MaxHeapSize – InitialHeapSize): The Second Virtual Space is actually the Difference between the Maximum Heap size (-Xmx)and the Initial Heap Size(-Xms). This is called as virtual space because initially the JVM will allocate the Initial Heap Size and then according to the requirement the Heap size can grow till the MaxHeapSize.

PermGen Space: (-XX:MaxPermSize): PermGen is a non-heap memory area where the Class Loading happens and the JVM allocates spaces for classes, class meta data,  java methods and the reference Objects here. The PermGen is independent from the Heap Area. It can be resized according to the requirement using -XX:MaxPermSize and -XX:PermSize  JVM Options. The Garbage collection happens in this area of JVM Memory as well. The Garbage collection in this area is called as “Class GC”. We can disable the Class Garbage Collection using the JVM Option -noclassgc. if  ”-noclassgc” Java Option is added while starting the Server. In that case the Classes instances which are not required will not be Garbage collected.

Native Area: Native Memory is an area which is usually used by the JVM for it’s internal operations and to execute the JNI codes. The JVM Uses Native Memory for Code Optimization and for loading the classes and libraries along with the intermediate code generation.

The Size of the Native Memory depends on the Architecture of the Operating System and the amount of memory which is already commited to the Java Heap. Native memory is an Process Area where the JNI codes gets loaded or JVM Libraries gets loaded or the native Performance packs and the Proxy Modules gets loaded.

There is no JVM Option available to size the Native Area. but we can calculate it approximately using the following formula:

NativeMemory = (ProcessSize – MaxHeapSize – MaxPermSize)