jvm-05. garbage collection

Garbage collection

What is rubbish

jvm in new Out object , When there is no reference to it , It's garbage .

Garbage location

Reference count

When the reference count changes to 0 It becomes garbage .

however , The reference count will have no way to locate the garbage object of circular reference ,
for example , There are three objects that reference each other ,
But no other external references point to them , They are all rubbish .
The garbage is also a pile of garbage when it is picked up （ laugh cry）.

Reachability analysis

Java This method is adopted

take "GC Roots" As a starting point , Start from these nodes and drill down to the referenced objects ,
Objects on the entire chain of references are marked as non garbage , The rest of the unmarked objects are garbage .

GC Roots： Thread stack local variables 、 Static variables 、 Variables of local method stack, etc .

Recycling of garbage

Three methods

Mark clear

Mark → And then remove

advantage ： Simple method , The efficiency is higher when there are many surviving objects ,
shortcoming ： Scan both sides , Low efficiency , It is also easy to produce fragments .
Because the reclaimed space will produce discontinuous memory .

Copy

Divide the space in two , Half of them are not used ,
During recycling, the surviving objects are neatly copied to the other half of the space .

advantage ： There are no fragments ,
shortcoming ： Waste space , Half the space is always free .

Tag compression

Mark → Then clean up the space when you clean it up

It is time consuming , The least efficient one , Because while cleaning up .

These three methods have their own characteristics , In practical application, it is generally used in combination .

Heap memory partition

Young people die in large numbers , Little survival , Recycle by copying ,
The survival rate of the older generation is high , Less recycling , use MS or MC, Mark removal and mark compression .
The recycling of Eden Park will have a feature , After a recycle, a large number of objects will be cleared ,
The recycling scope includes Eden Park and the current survivor area , experienced minor GC Surviving objects , Generational age will +1,
Generation age does not reach a certain value , for example 15, Will be copied to another survivor zone , Otherwise, move to the old age .

The object of these situations will be put into the old age （ The allocation of objects ）