Java GC: why two survivor regions?

Question

For Sun/Oracle\'s JVM, I\'ve read that the GC algo divides new generation into one Eden region and two survivor regions. What I\'m wondering about is, why two survivor regio

Answer 1

All the current answer talk about memory fragmentation, this is also another reason to have generations in GC.

The runtime records all “old objects” that point to “new objects”, this is done every time a “pointer” field is updated. Then when a “minor” GC is done, only “new” objects need to be scanned.

Over the years it has been found that just have “new” and “old” is not enough, and it is good to have a 3rd generation that is “middle aged”.

Answer 2

The role of two survivor spaces gets reversed after the operation of a minor garbage collection

The two survivor spaces. These hold objects that have survived at least one minor garbage collection but have been given another chance to become unreachable before being promoted to the old generation. Only one of them holds objects, while the other is most of the time unused.

During the operation of a minor garbage collection, objects that have been found to be garbage will be marked. Live objects in the eden that survive the collection are copied to the unused survivor space. Live objects in the survivor space that is in use, which will be given another chance to be reclaimed in the young generation, are also copied to the unused survivor space. Finally, live objects in the survivor space that is in use, that are deemed “old enough,” are promoted to the old generation.

At the end of the minor garbage collection, the two survivor spaces swap roles. The eden is entirely empty; only one survivor space is in use; and the occupancy of the old generation has grown slightly. Because live objects are copied during its operation, this type of garbage collector is called a copying garbage collector.

Source : above are the excerpts from page 83 of Java Performance, by Charlie Hunt and Binu John.

Answer 3

Two survivors are implementation of mark and copy algorithm. These are used in GC for younger generation. As mentioned by Ryan in option 3 here

Answer 4

What are the advantages and disadvantages of copying all instances of a generation from one space to another, versus copying them in memory-address order to the start of a generation's space? Processing items in order would probably require adding an extra pointer per item, but would eliminate the need for one of the 'survivor' spaces.

Answer 5

Heap Memory in Java Java objects created in an area called the heap memory.The heap memory is created when the JVM starts up, heap memory is increased or decreased when an java application running. When the heap memory becomes full, garbage collector removes the unused objects, thus garbage collector makes a space for new objects.

The heap memory is divided into two areas (or generations) called

1.young space. 2.old space.

1.In young space, there is Eden space for new Object and there are two Survivor Spaces(from and to), these two Survivor Spaces are always same size.

2.Survivor Spaces are used to store survival Objects.When the young space becomes full, garbage collector removes the unused objects by running a special young collection, where all objects that have lived long enough in the young space are promoted (moved) to the old space,thus freeing up the young space for more object allocation.

3.If Eden space is full,GC will run, if any objects are live in this Eden space, those are moved to Survivor Space.

4.In young space, GC normally use Copying Algorithm, which is fast, Everytime, survival Objects are copied to one of the Survivor Space.

5.If Survivor Space is full, rest of live Objects are directly copied to Old space.

6.In Old space, GC nornally use Mark-Compact Algorithm, which is slow but requires less memory.

7.When the old space becomes full garbage is collected there, a process called an old collection.In Old space, long live time Objects stay there.

8.Out of Memory will happen there is no space for new Object even GC done for OLD or Perm part.

9.Object is moved during Garbage Collection: eden -> survivor -> tenured(old space)

Answer 6

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

Eden space : New objects will be allocated in the memory pool. The assumption is that most object get dereferenced and become unreachable soon after their creation. Objects not being dereferenced will be copied by the new generation garbage collector into the survivor spaces. They may get copied n some special cases directly into the old generation pool.

Survivor spaces: These two small spaces keep the surviving objects of a young generation garbage collection. Surviving objects will be copied for a (small) number of times from one survivor into the other. This allows to harvest our more dereferenced objects.

Old generation: The largest memory pool which should keep the long living objects. Objects are getting copied into this pool once they leave the survivor spaces.

Permament generation: This fairly unknown pool keeps the information of all the classes. It doesn't need any attention for most applications. It may need to be adapted for some applications with many classes. It may need some attentention as well if the application permanently loads and unloads classes.

Other advantages:

• Memory fragmentation
• It improves GC performance

Please find the following links for more details which can help you understand more

http://www.scalingbits.com/javaprimer

http://java.sys-con.com/node/84695