JVM foundation ＞ G1 garbage collector

say something G1 Garbage collector

G1 overview

G1 GC The full name is Garbage First Garbage Collector, Garbage priority garbage collector , hereinafter referred to as G1.

• G1 yes HotSpot JVM Of Short pause garbage collector .

• In fact, about G1 My thesis dates back to 2004 In, there was , however G1 Is in 2012 year 4 Released on JDK 7u4 It's only in the middle of the world .

• In the long run ,G1 Aimed at replace CMS（Concurrent Mark Sweep） Garbage collector .

• G1 from JDK9 Started as Default garbage collector .

• If pause time is more important to an application than throughput ,G1 It's a very suitable choice .

On the whole G1 It has the following characteristics ：

• G1 Still generational （ The younger generation , Old age ） The garbage collector
• G1 Two garbage collection algorithms are implemented -> Mark - Copy algorithm + Mark - Sorting algorithm
• G1 There is no need to recycle all the garbage in the heap at one time ,G1 A piece of memory with too much garbage will be dynamically selected for priority recycling
• And you can specify the desired pause time

G1 Innovative design of waste recycling

Memory layout

G1 Although it is still divided into generations （ The younger generation , Old age ） The garbage collector , But it no longer follows the generational arrangement of objects in the heap , Instead, the heap is divided into several equal sized areas .

But to become ：

Equalize heap memory into small chunks (Region), This little piece (Region) How big is controllable

Through parameters -XX:G1HeapRegionSize=n You can specify the partition size (1MB~32MB, And it must be 2 The power of )

By default, the whole heap is divided into 2048 Zones -> If the heap is 2g, So every little piece (Region) Namely 1M

Humongous： When you allocate more than half of an object area (Region) The size of ( exceed 0.5M when ), This object will be placed in this area . This area belongs to the old age area .

G1 Garbage collection process

G1 Garbage collection mainly includes 2 individual ：

• Cenozoic recycling process （young gc） -> Recycling
  • Applications allocate memory , When the new generation Eden When the zone is exhausted, the new generation recycling process begins
• Mixed recycling process （mixed gc） -> Recycling
  • When the heap memory usage reaches a certain value （ Default 45%） when , Start the old age concurrent tagging process , Mark complete and start the mixed recycling process immediately .

Cenozoic recycling process

The general logic will not change much : The living object is transferred to a / Or more survivor Put it on the block . If the survival time reaches the threshold , This part of the object will be promoted to the old age .

Mixed recycling process

1. Initial marker
   Just mark GC roots Objects that can be directly related to . This step requires STW, But the time is very short .

2. Concurrent Tags
   from GC roots Using reachability algorithms , Mark objects in heap memory （ Tricolor notation ）, Recursively scans the entire heap . This step takes too long , It is executed concurrently with the user business program .

3. Re label
   For user threads For a short time STW, Used to handle the finalization of some changes to objects in concurrent markup .

4. Screening and recovery
   G1 According to the pause time specified by the user , Maximum recycling region, Will need to be recycled region Copy to blank region in , Then put the original region All recycled . The Steps also require STW, Because it involves the movement of objects （ One region To another region）.