How is the java memory pool divided?

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I’m currently monitoring a Java application with jconsole. The memory tab lets you choose between:

Heap Memory Usage
Non-Heap Memory Usage
Memory Pool “Eden Space”
Memory Pool “Survivor Space”
Memory Pool “Tenured Gen”
Memory Pool “Code Cache”
Memory Pool “Perm Gen”

What is the difference between them ?

pool memory java
5 Answers

Heap memory

The heap memory is the runtime data area from which the Java VM allocates memory for all class instances and arrays. The heap may be of a fixed or variable size. The garbage collector is an automatic memory management system that reclaims heap memory for objects.

  • Eden Space: The pool from which memory is initially allocated for most objects.

  • Survivor Space: The pool containing objects that have survived the garbage collection of the Eden space.

  • Tenured Generation or Old Gen: The pool containing objects that have existed for some time in the survivor space.

Non-heap memory

Non-heap memory includes a method area shared among all threads and memory required for the internal processing or optimization for the Java VM. It stores per-class structures such as a runtime constant pool, field and method data, and the code for methods and constructors. The method area is logically part of the heap but, depending on the implementation, a Java VM may not garbage collect or compact it. Like the heap memory, the method area may be of a fixed or variable size. The memory for the method area does not need to be contiguous.

  • Permanent Generation: The pool containing all the reflective data of the virtual machine itself, such as class and method objects. With Java VMs that use class data sharing, this generation is divided into read-only and read-write areas.

  • Code Cache: The HotSpot Java VM also includes a code cache, containing memory that is used for compilation and storage of native code.

Here's some documentation on how to use Jconsole.

user_892660

The Heap is divided into young and old generations as follows :

Young Generation: It is a place where an object lived for a short period and it is divided into two spaces:

  • Eden Space: When object created using new keyword memory allocated on this space.
  • Survivor Space (S0 and S1): This is the pool which contains objects which have survived after minor java garbage collection from Eden space.

Old Generation: This pool basically contains tenured and virtual (reserved) space and will be holding those objects which survived after garbage collection from the Young Generation.

  • Tenured Space: This memory pool contains objects which survived after multiple garbage collection means an object which survived after garbage collection from Survivor space.

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Explanation

Let's imagine our application has just started.

So at this point all three of these spaces are empty (Eden, S0, S1).

Whenever a new object is created it is placed in the Eden space.

When the Eden space gets full then the garbage collection process (minor GC) will take place on the Eden space and any surviving objects are moved into S0.

Our application then continues running add new objects are created in the Eden space the next time that the garbage collection process runs it looks at everything in the Eden space and in S0 and any objects that survive get moved into S1.

PS: Based on the configuration that how much time object should survive in Survivor space, the object may also move back and forth to S0 and S1 and then reaching the threshold objects will be moved to old generation heap space.

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