Deep understanding of JVM (V) - garbage collection (II)

In depth understanding of JVM（ 5、 ... and ）—— Garbage collection （ Two ）

2、 Garbage collection algorithm

Mark - eliminate

Definition ： As the name suggests , It refers to the process of garbage collection in the virtual machine , Firstly, the recyclable object is determined by marking algorithm , Then the garbage collector clears the corresponding content according to the identification , Make room for heap memory

• Freeing up memory space here does not clear bytes of memory space 0, Instead, record the start and end address of this memory , The next time you allocate memory , Will directly Cover This memory

shortcoming ： It is easy to produce a large number of memory fragments , May not be able to meet the memory allocation of large objects , Once the object cannot be allocated , That would lead to jvm start-up gc, Once started gc, Our application will pause , This causes the response speed of the application to slow down

Mark - Arrangement

Mark - Arrangement Will not be GC Root Referenced object recycling , Know the memory space it occupies . Then sort out the remaining objects , It can effectively avoid problems caused by memory fragmentation , But because the whole takes some time , So it's less efficient

Copy

Divide the memory into two areas of equal size ,FROM and TO（TO Empty in middle ）. First will be GC Root The referenced object is from FROM Put in TO in , Recycling is not GC Root Referenced object . Then exchange FROM and TO. This can also avoid the problem of memory fragmentation , But it takes up double the memory space .

3、 Generational recycling

Recycling process

The newly created objects are placed in The new generation of Eden in

When there is not enough memory in the garden of Eden , There will be a garbage collection , The recycling at this time is called Minor GC

Minor GC Will The garden of Eden and the survival zone FROM Living objects First Copied to the Survival zone TO in , And let it Life plus 1, Again Swap two surviving areas

Create the object again , If the new generation of Eden is full again , will Trigger again Minor GC（ Will trigger stop the world, Pause other user threads , Just let the garbage collection thread work ）, At this time, we will not only recycle the garbage in the garden of Eden , And recycle the garbage in the surviving areas , Then copy the active object to the surviving area TO in . After recycling, two surviving areas will be exchanged , And let the objects in the surviving area Life plus 1

If the object in the surviving area Life exceeds a certain threshold （ The maximum is 15,4bit）, Will be Put in the older generation in

If the memory of the new generation and the old generation is full , It will trigger first Minor GC, Trigger again Full GC, scanning In the new generation and the old generation All objects that are no longer in use and recycle

GC analysis

Large object processing strategy

When I meet a A larger object when , Even the new generation The garden of Eden It's empty , also The object cannot be accommodated when , The object will be Directly promoted to the elderly generation

Thread memory overflow

A thread's memory overflowed and threw an exception （out of memory）, Won't let other threads end

This is because when a thread Throw out OOM After abnormality , All the memory resources it occupies will be released , This will not affect the operation of other threads , The process is still normal