JVM garbage collection mechanism, where does GC occur in JVM, how many types of GC, and what are their algorithms?

What is GC?

Generational collection algorithm

1) Minor GC: Young area is collected frequently in number of times

2) Full GC: Old area is collected less frequently

3) Basically do not move the Perm area

GC occurs in the heap

 

GC algorithm

1) Reference counting method (has been deprecated)

 

2) Copying algorithm (Copying)

Occurs in the young generation , using Minor GC

principle:

Starting from the root set (GC Root), through Tracing from

Find the surviving object in From and copy it to To

From and To exchange identities, the next memory allocation starts from To

 

advantage:

1) No marking and removal process, high efficiency

2) There is no memory fragmentation , you can use bump-the-pointer to achieve fast memory allocation

Disadvantages:

Need double space

 

3) Mark-Sweep

Happened in the old age

标记: Start scanning from the root collection and mark the surviving objects

清除: Scan the entire memory space, reclaim unmarked objects, use free-list to record the available areas

advantage

No extra space required

Disadvantage

1) Two scans are time-consuming

2) Memory fragmentation will occur

 

 

 

4) Mark-Compact

Happened in the old age

principle

1) Mark: Same as mark-clear

2) Compression: Scan twice and slide the surviving object to one end

advantage

No memory fragmentation

Disadvantage

The cost of moving objects

 

5) Mark-Sweep-Compact

Actually 3) and 4) are used in combination