Java (JVM) Memory Structure
JVM memory is divided into multiple parts: Heap Memory, Non-Heap Memory, and Other.
Heap memory
Heap memory is the run time data area from which the memory for all java class instances and arrays is allocated. The heap is created when the JVM starts up and may increase or decrease in size while the application runs. The size of the heap can be specified using –Xms VM option. The heap can be of fixed size or variable size depending on the garbage collection strategy. Maximum heap size can be set using –Xmx option. By default, the maximum heap size is set to 64 MB.
Non-Heap memory
The JVM has memory other than the heap, referred to as Non-Heap Memory. It is created at the JVM startup and stores per-class structures such as runtime constant pool, field and method data, and the code for methods and constructors, as well as interned Strings. The default maximum size of non-heap memory is 64 MB. This can be changed using –XX:MaxPermSize VM option.
Other memory
JVM uses this space to store the JVM code itself, JVM internal structures, loaded profiler agent code and data, etc.
The JVM heap is physically divided into two parts (or generations): nursery (or young space/young generation) and old space (or old generation).
The nursery is a part of the heap reserved for allocation of new objects. When the nursery becomes full, garbage is collected by running a special young collection, where all the objects that have lived long enough in the nursery are promoted (moved) to the old space, thus freeing up the nursery for more object allocation. This garbage collection is called Minor GC. The nursery is divide into three parts – Eden Memory and two Survivor Memory spaces.
Important points about the nursery space:
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 moves them to the other survivor space. So at a time, one of the survivor space is always empty
Objects that have survived many cycles of GC, are moved to the old generation memory space. Usually it is done by setting a threshold for the age of the nursery objects before they become eligible to promote to old generation
When the old generation becomes full, garbage is collected there and the process is called as old collection. 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 as Major GC and usually takes longer time. The reasoning behind a nursery is that most objects are temporary and short lived. A young collection is designed to be swift at finding newly allocated objects that are still alive and moving them away from the nursery. Typically, a young collection frees a given amount of memory much faster than an old collection or a garbage collection of a single-generational heap (a heap without a nursery).
Recent releases include a part of nursery called as keep area and it is reserved. The keep area contains the most recently allocated objects in the nursery and is not garbage collected until the next young generation. This prevents objects from being promoted just because they were allocated right before a young collection is started.
JVM memory is divided into multiple parts: Heap Memory, Non-Heap Memory, and Other.
Heap memory
Heap memory is the run time data area from which the memory for all java class instances and arrays is allocated. The heap is created when the JVM starts up and may increase or decrease in size while the application runs. The size of the heap can be specified using –Xms VM option. The heap can be of fixed size or variable size depending on the garbage collection strategy. Maximum heap size can be set using –Xmx option. By default, the maximum heap size is set to 64 MB.
Non-Heap memory
The JVM has memory other than the heap, referred to as Non-Heap Memory. It is created at the JVM startup and stores per-class structures such as runtime constant pool, field and method data, and the code for methods and constructors, as well as interned Strings. The default maximum size of non-heap memory is 64 MB. This can be changed using –XX:MaxPermSize VM option.
Other memory
JVM uses this space to store the JVM code itself, JVM internal structures, loaded profiler agent code and data, etc.
The JVM heap is physically divided into two parts (or generations): nursery (or young space/young generation) and old space (or old generation).
The nursery is a part of the heap reserved for allocation of new objects. When the nursery becomes full, garbage is collected by running a special young collection, where all the objects that have lived long enough in the nursery are promoted (moved) to the old space, thus freeing up the nursery for more object allocation. This garbage collection is called Minor GC. The nursery is divide into three parts – Eden Memory and two Survivor Memory spaces.
Important points about the nursery space:
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 moves them to the other survivor space. So at a time, one of the survivor space is always empty
Objects that have survived many cycles of GC, are moved to the old generation memory space. Usually it is done by setting a threshold for the age of the nursery objects before they become eligible to promote to old generation
When the old generation becomes full, garbage is collected there and the process is called as old collection. 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 as Major GC and usually takes longer time. The reasoning behind a nursery is that most objects are temporary and short lived. A young collection is designed to be swift at finding newly allocated objects that are still alive and moving them away from the nursery. Typically, a young collection frees a given amount of memory much faster than an old collection or a garbage collection of a single-generational heap (a heap without a nursery).
Recent releases include a part of nursery called as keep area and it is reserved. The keep area contains the most recently allocated objects in the nursery and is not garbage collected until the next young generation. This prevents objects from being promoted just because they were allocated right before a young collection is started.
No comments:
Post a Comment