Understanding Heap Memory
Heap memory is a part of memory allocated to JVM which is shared by all executing threads in the application. It is the part of JVM in which all class instances and are allocated. It is created on the Start-up process of JVM. It does not need to be contiguous and its size can be static or dynamic. Space allocated to the memory is reclaimed through an automatic memory management process called garbage collection. Heap memory is a shared area which is utilized during runtime of java applications. It is created during the instantiation of Java Virtual Machine (JVM).
This memory is shared by instances of all the classes created during the runtime of an application. As per system configuration, the size of heap memory may be fixed or variable. In order to reclaim the space of heap memory an automatic memory management process called garbage collection is triggered by JVM. JVM provides control to developers to vary the size of heap memory according to requirement.
As you can see in the above image, JVM memory is divided into separate parts. At broad level, JVM Heap memory is physically divided into two parts – Young Generation and Old Generation.
Young Generation – Heap Memory
The young generation is the place where all the new objects are created. When the young generation is filled, garbage collection is performed. This garbage collection is called Minor GC. Young Generation is divided into three parts – Eden Memory and two Survivor Memory spaces.
Important Points about Young Generation Spaces:
- Most of the newly created objects are located in the Eden memory space.
- When Eden space is filled with objects, Minor GC is performed and all the survivor objects are moved to one of the survivor spaces.
- Minor GC also checks the survivor objects and move them to the other survivor space. So at a time, one of the survivor space is always empty.
- Objects that are survived after many cycles of GC, are moved to the Old generation memory space. Usually, it’s done by setting a threshold for the age of the young generation objects before they become eligible to promote to Old generation.
Old Generation – Heap Memory
Old Generation memory contains the objects that are long-lived and survived after many rounds of Minor GC. Usually, garbage collection is performed in Old Generation memory when it’s full. Old Generation Garbage Collection is called Major GC and usually takes a longer time.
Stop the World Event
All the Garbage Collections are “Stop the World” events because all application threads are stopped until the operation completes.
Since Young generation keeps short-lived objects, Minor GC is very fast and the application doesn’t get affected by this.
However, Major GC takes a long time because it checks all the live objects. Major GC should be minimized because it will make your application unresponsive for the garbage collection duration. So if you have a responsive application and there are a lot of Major Garbage Collection happening, you will notice timeout errors.
The duration taken by garbage collector depends on the strategy used for garbage collection. That’s why it’s necessary to monitor and tune the garbage collector to avoid timeouts in the highly responsive applications.
Code Cache – Heap Memory
Code Cache (Virtual or reserved) : If you are using HotSpot Java VM this includes code cache area that containing memory which will be used for compilation and storage of native code.
Permanent Generation – Heap Memory
Permanent Generation or “Perm Gen” contains the application metadata required by the JVM to describe the classes and methods used in the application. Note that Perm Gen is not part of Java Heap memory.
Perm Gen is populated by JVM at runtime based on the classes used by the application. Perm Gen also contains Java SE library classes and methods. Perm Gen objects are garbage collected in a full garbage collection.
Java8 Update: PermGen is replaced with Metaspace which is very similar.
Main difference is that Metaspace re-sizes dynamically i.e., It can expand at runtime.
Java Metaspace space: unbounded (default)