Java heap의 세가지 영역

출처:  http://www.vimaljava.com/memory-management/understanding-java-memory-structure/ 
 

Understanding Java Memory Structure

One strength of the java platform is that it shields the developer from the complexity of memory allocation and garbage collection.  We all know that java stores objects in a Heap memory.  Internally Java partitions the heap memory space logically into three areas called generations.

1. The Young Generation
2. The Tenured Generation
3. The Permanent Generation

Out if these three partitions, only the young and tenured generation spaces are used for allocating memory to objects. The permanent generation space is used for holding the data needed by the virtual machine to describe objects that do not have equivalence at the Java language level.

This partitioning is done in order to improve the performance of garbage collection as performing garbage collection on the entire heap will be very expensive. Instead the architects of java decided to partition the heap space into three parts.

The Young Generation

The young generation is where space is allocated for newly created objects.  In most applications, most of the objects created are used and referenced only for a very short span of time (high infant mortality rate), for example an iterator instance is discarded as soon as the loop is complete.

The purpose of having a separate space of young generation is to maximize promptness (the time between when an object becomes dead and when the memory becomes available) and to reduce the overhead of scanning the entire heap during every garbage collection process.

Garbage collection in the young generation happens when the space in this generation fills up and is called the minor collection. Minor collections can be optimized assuming a high infant mortality rate.  It is well-tuned in the sense that the young generation is large enough (and thus the period between minor collections long enough) that the minor collection can take advantage of the high infant mortality rate. The time required for garbage collection is directly proportional to the number of live objects.  A young generation full of dead objects is collected very quickly. Some surviving objects are moved to a tenured generation.

The Tenured Generation

Objects which survive the young generation are eventually moved into the tenured generation. Garbage collection in the tenured generation happens when the space in the tenured generation fills up and is called the major collection. Since the tenured generation usually have a large number of alive objects, the time required for garbage collecting the tenured generation is much higher than that for the young generation. Hence, garbage collection of the tenured generation is done at a much lesser rate than the young generation.

The Permanent Generation

The permanent generation is not used for allocating objects. Instead it is used for holding the data needed by the virtual machine to describe objects that do not have equivalence at the Java language level. For example objects describing classes and methods are stored in the permanent generation.


아래의 pdf파일은  Fillip Hanik이라는 개발자의 Inside the Java Virtual Machine: Memory Management and Troubleshooting이라는 제목의 슬라이드로 위의 내용을 이해하는데 도움이 된다.

출처:  http://www.springsource.com/files/uploads/all/pdf_files/news_event/Inside_the_JVM.pdf

Inside_the_JVM.pdf