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JVM memory area and garbage collection

2020-11-06 01:18:00 【Clamhub's blog】

1、JAVA Memory area and memory overflow

1.1、 summary

Java in JVM Memory management mechanism is provided ,Java Virtual machine is executing Java During the process of the program, the memory is divided into different data areas , Pictured :

1.2、 Program counter

Program counter is the line number indicator of bytecode executed by the current thread , The function is to get the next bytecode instruction to be executed according to the counter value . When it comes to java Method , It records the address of the executing virtual machine bytecode instruction , If it is Native Method , Then the value of this counter is null . There is no such thing as OutOfMemoryError.

1.3、 Virtual machine stack

Every ordinary Java Method （ remove Native Method ） The stack frame will be created at the same time during execution , Used to store local variables 、 Stack operation 、 Dynamic links 、 Method exit information , Each method is called until the completion of the process corresponding to the stack frame in JVM Stack in and stack out . The memory space required by the local variable table is allocated among compilers .

There are two types of exceptions associated with the virtual machine stack ：

StackOverflowError

The stack depth of thread request is greater than the maximum depth allowed by virtual machine .

OutOfMemoryError

The virtual machine stack cannot be extended to enough memory .

1.4、 Native Method Stack

For virtual machine execution Native Method , The others are the same as the local method stack . There will be StackOverflowError and OutOfMemoryError.

1.5、 Pile up

Create a heap after the virtual machine starts , For storing object instances . The heap is the main working area of the garbage collector , Mainly divided into the new generation and the old generation , The new generation can be subdivided into Eden Space 、From Survivor Space 、To Survivor Space .java Program startup , You can use -Xmx And -Xms Control the size of the heap . If there is no memory in the heap to complete the instance allocation and the heap cannot be expanded, it will be thrown OutOfMemoryError.

1.6、 Method area

The method area mainly stores the metadata of the class , Such as class information loaded by virtual machine 、 Constant 、 Static variables 、 Real time compiler compiled code ,JDK1.8 It was realized by permanent replacement ,JDK1.8 Then use Metaspace , And Metaspace uses system memory . If you can't request memory , Will throw out OutOfMemoryError.

2、 Garbage collection

2.1、 How to judge that the object has died ？

2.1.1、 Reference counting

Add a reference counter to the object , When there's a place to quote , Counter value +1, When the reference fails , Counter value -1. The counter for 0 The object of death is , But there's a problem ： Object circular reference , Two objects refer to each other , The reference counter that causes them is not 0, So I can't tell GC Recycling .

2.1.2、GC Roots Search for

Java It is used in GC ROOT Search for , The idea is to go through a series called “GC Roots” As the starting point , Start with these nodes and drill down , The path in search is called the reference chain , When GC Roots When you can't reach an object , The object is not available .

GC Roots These include the following ：

Objects referenced in local variable table in stack frame
Object referenced by a class static property in a method area
The object referenced by a constant in the method area
Native Method Stack Native Method reference object

2.2、 Garbage collection algorithm

2.2.1、 Mark - Clear algorithm

Mark - The clearing algorithm is divided into two stages ：

Mark ： First mark all objects to be recycled .
eliminate ： Uniformly reclaim marked objects .

This algorithm has two disadvantages ：

The efficiency is not high
It will generate discontinuous memory fragments

2.2.2、 Copy algorithm

The replication algorithm is very efficient , It divides the available memory into two equal sized blocks according to the capacity , Use only one piece at a time , When one piece is used up , Copy the surviving objects to another piece , Then clear the used memory space .

This algorithm is very suitable for recycling the new generation , In the new generation, it is divided into Eden Space 、From Survivor Space 、To Survivor Space , Generally, the proportion of allocated memory is 8：1：1, When recycling , take Eden And From Survivor Objects that are still alive in are copied to To Survivor in , Then clean up Eden And From Servivor Space , When To Survivor When there is not enough space , Need to rely on the old age .

2.2.3、 Mark - Sorting algorithm

In the old age , The survival rate of the subjects is relatively high , So mark - The sorting algorithm has been proposed , First mark the objects to recycle , Then move all living objects to one end , And then clean up the dead ：

2.2.4、 Generational collection algorithm

The idea of generational recycling is based on the life cycle of the object , Divide different memory into several pieces , According to the characteristics of each block of memory, the appropriate collection algorithm , For example, the new generation uses replication algorithm , The old days use signs - Sorting algorithm .

2.3、 Garbage collector

The following figure shows 7 Different generations of collectors , If there is a connection between the two collectors , Can be used with .

You can view the garbage collector information through the following command ：

1	java -XX:+PrintCommandLineFlags -version

My test server results ：

-XX:InitialHeapSize=524503488 -XX:MaxHeapSize=8392055808 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
java version "1.8.0_152"
Java(TM) SE Runtime Environment (build 1.8.0_152-b16)
Java HotSpot(TM) 64-Bit Server VM (build 25.152-b16, mixed mode)

It can be seen that ：ParallelGC.

JVM Parameter correspondence ：

Here is a brief introduction to 7 A garbage collector ：

2.3.1、Serial The collector

A new generation of single-threaded collectors , A simple and efficient .

2.3.2、ParNew The collector

Serial Multithreaded version of collector , In addition to multithreading in garbage collection , The rest go with Serial The collectors are the same .

2.3.3、Parallel Scavenge The collector

Parallel Scavenge Collector is a new generation of parallel collector which adopts replication algorithm , Focus on the throughput of the system , Tasks that are suitable for background computing without too much user interaction .

2.3.4、Serial Old The collector

Serial Old It's a single threaded old-fashioned garbage collector , Use the tag - Sorting algorithm . It is simple and efficient .

2.3.5、Parallel Old The collector

Parallel Old The collector is Parallel Scanenge The old version , Multithreaded garbage collection , It's also marked - Sorting algorithm .

2.3.6、CMS The collector

CMS Focus on service response time , It's based on markers - Clear algorithm implementation . With concurrent collection 、 The characteristic of a low pause .

2.3.7、G1 The collector

Garbage First, Based on tags - Sorting algorithm , It will Java Pile up （ Including the new generation and the old generation ） It is divided into several independent areas of fixed size , And keep track of the amount of garbage in these areas , Maintain a priority list in the background , Each time according to the allowed collection time , Priority to recycle the most garbage .