New generation garbage collector ZGC

Catalog

1、 What is? ZGC

2、ZGC Design purpose

3、ZGC present situation

4、ZGC Core technologies

4.1  Multiple mapping

4.2  Dye pointer

4.3 Reading barrier

---

1、 What is? ZGC

ZGC（The Z Garbage Collector） yes JDK 11 Launched in the pursuit of an ultimate low latency experimental garbage collector .

ZGC It was recently by Oracle by OpenJDK New open source garbage collector . It is mainly composed of Per Liden To write .ZGC Be similar to Shenandoah or Azul Of C4, Focus on reducing pause times while still compressing the heap .

2、ZGC Design purpose

ZGC The core of is a concurrent garbage collector , The goal of its design is ：

1)  The pause time shall not exceed 10ms;

2)  The pause time doesn't vary with the size of the heap , Or the size of the active object ;

3)  The supported heap range is 8MB~4TB Level （ Future support 16TB）.

All in all , ZGC The purpose of is to reduce the pause time at the same time , Can still compress the heap .

3、ZGC present situation

Up to date JDK15 Look at ,“ The pause time shall not exceed 10ms” and “ Support 16TB A pile of ” These two goals have been achieved , And the official made it clear that JDK15 Medium ZGC No longer an experimental garbage collector , And it is suggested to put into production .

4、ZGC Core technology of Technique

4.1  Multiple mapping

Let's first look at an interesting example .

You are a son in the eyes of your parents , In your girlfriend's eyes, it's a boyfriend . In front of people all over the world is the most handsome person . You have another name , But the name is just your code name , It's not you . If your name is the only one in the world , adopt “ What's your name ”、“ Your father's son ”、“ Your girlfriend's boyfriend ”,“ The most handsome man in the world ” It's all about you in the end .

Now let's see ZGC Memory management for .

ZGC The basic unit for managing physical memory in is segment.segment Default and small page size equally , All are 2MB. introduce segment To avoid frequent system calls to request and free memory , An application 2MB, When segment Idle , Will be added to the free list , Wait and reuse .

ZGC In order to be efficient 、 Flexible memory management , Two level memory management is realized ： Virtual memory and physical memory , And the mapping between physical memory and virtual memory is realized . This is basically consistent with the design idea of virtual address and physical address in the operating system .

When an application creates an object , First, apply for a virtual address in heap space ,ZGC At the same time, for the object in Marked0、Marked1 and Remapped Each of the three view spaces applies for a virtual address , And these three virtual addresses correspond to the same physical address .

stay ZGC There is only one valid space among the three spaces at the same time .

Why is it so designed ？

This is it. ZGC The brilliance of , Use virtual space for time .

The switching of these three spaces is triggered by different stages of garbage collection , By limiting three spaces to have and only one space at the same time, it can be completed effectively and efficiently GC Concurrent operation of process , This and ZGC Concurrent processing algorithm is related .ZGC The concurrent processing algorithm makes use of the switch between global spatial view and object address view , combination SATB The algorithm achieves efficient concurrency .

4.2  Dye pointer

We all know , Previous garbage collectors used to put GC Information （ Tag information 、GC Generational age ..） There is an object header Mark Word in .

and ZGC Here's how it works ：

If an object is a garbage object . Mark this object as garbage in the information of this object . No matter where this object is used in the future , We all know that he is a garbage object .

ZGC Store the object information in the pointer , This technology is called —— Dye pointer （Colored Pointer）.

stay 64 In a bit machine , The object pointer is 64 Bit .

ZGC Use 64 Bit address space 0~43 Bit storage object address ,2^44 = 16TB, therefore ZGC The biggest support 16TB A pile of .

And the first 44~47 Bit as a color marker bit ,Marked0、Marked1 and Remapped Represents three view flags ,Finalizable Indicates that this object can only be accessed through finalizer Ability to visit .

The first 48~63 The bit is fixed to 0 There is no use .

4.3 Reading barrier

The reading barrier is JVM The technique of inserting a small piece of code into application code . When an application thread reads an object reference from the heap , It will execute this code . Don't compare this barrier with Java The read barrier in the memory model is confused , The two are not the same thing at all ,ZGC The reading barrier is more like AOP technology , Add an extra processing to the read operation at the bytecode level or at the compile code level .

ZGC As an excellent garbage collector , It draws on Pauseless GC, But in G1 It's not popular today , talk about ZGC It seems too early . But maybe we're not talking about ZGC, It is ZGC The design idea behind it .

Official account + Input [ Interview questions ] + Get the interview materials free of charge ( Interview outline + Interview answers )!