How is the java memory pool divided

Question

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

User · Answer

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.

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.

User · Answer

Java Heap Memory is part of memory allocated to JVM by Operating System   Objects reside in an area called the heap  The heap is created when the JVM starts up and may increase or decrease in size while the application runs  When the heap becomes full  garbage is collected     You can find more details about Eden Space  Survivor Space  Tenured Space and Permanent Generation in below SE question   Young   Tenured and Perm generation  PermGen has been replaced with Metaspace since Java 8 release   Regarding your queries    Eden Space  Survivor Space  Tenured Space are part of heap memory Metaspace and Code Cache are part of non-heap memory    Codecache  The Java Virtual Machine  JVM  generates native code and stores it in a memory area called the codecache  The JVM generates native code for a variety of reasons  including for the dynamically generated interpreter loop  Java Native Interface  JNI  stubs  and for Java methods that are compiled into native code by the just-in-time  JIT  compiler  The JIT is by far the biggest user of the codecache

User · Answer

The new keyword allocates memory on the Java heap  The heap is the main pool of memory  accessible to the whole of the application  If there is not enough memory available to allocate for that object  the JVM attempts to reclaim some memory from the heap with a garbage collection  If it still cannot obtain enough memory  an OutOfMemoryError is thrown  and the JVM exits  The heap is split into several different sections  called generations  As objects survive more garbage collections  they are promoted into different generations  The older generations are not garbage collected as often  Because these objects have already proven to be longer lived  they are less likely to be garbage collected  When objects are first constructed  they are allocated in the Eden Space  If they survive a garbage collection  they are promoted to Survivor Space  and should they live long enough there  they are allocated to the Tenured Generation  This generation is garbage collected much less frequently  There is also a fourth generation  called the Permanent Generation  or PermGen  The objects that reside here are not eligible to be garbage collected  and usually contain an immutable state necessary for the JVM to run  such as class definitions and the String constant pool  Note that the PermGen space is planned to be removed from Java 8  and will be replaced with a new space called Metaspace  which will be held in native memory  reference http   www programcreek com 2013 04 jvm-run-time-data-areas

User · Answer

With Java8  non heap region no more contains PermGen but Metaspace  which is a major change in Java8  supposed to get rid of out of memory errors with java as metaspace size can be increased depending on the space required by jvm for class data

User · Answer

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

[java] How is the java memory pool divided?

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