Explore ZGC

ZGC（The Z Garbage Collector） yes JDK 11 A low latency garbage collector introduced in , It's at the experimental stage , Its advantages include ：

• The pause time shall not exceed 10ms, Can be in KB~TB Garbage collection under the scope of work ;
• The pause time doesn't vary with the size of the heap , Or the size of the active object ;
• Support 8MB~4TB Level heap （ Future support 16TB）

Why introduce ZGC

It is well known that in some applications with high performance and low latency ,GC Often the bottleneck of application , Because in GC In the meantime STW（Stop The World）, When STW when , All application threads stop active , wait for GC The pause ended . In some systems with strict response time ,GC It is often very difficult to optimize , This is also an important reason why garbage collectors are evolving .

ZGC Guarantee , In any case , The delay will not exceed 10 millisecond .

The Z Garbage Collector, also known as ZGC, is a scalable low latency garbage collector designed to meet the following goals:

• Pause times do not exceed 10ms
• Pause times do not increase with the heap or live-set size
• Handle heaps ranging from a few hundred megabytes to multi terabytes in size

Now for CMS and G1 Of GC Process and pause time bottlenecks , Globally concurrent ZGC（ Mark 、 The transition and relocation phases are almost all concurrent ） Obviously a better choice .

ZGC Characteristics

ZGC The most typical feature is that it is a concurrent (concurrent) Of GC, Other features are as follows ：

• It can mark memory , Replication and migration (relocate) Memory , All operations are concurrent , At the same time, it has a concurrent reference processor

• Other garbage collectors use store barriers,ZGC Use load barriers, Used to track memory

  • lock->unlock->read->load Read memory
  • use->assign->store->write Write memory

• ZGC You can configure the size and policy more flexibly , Compared with G1, It can better handle very large (very large) Release of objects

• ZGC Only one generation , No Cenozoic , The old generation , however ZGC It can support local compression , In memory recovery and migration (reclaim and relocate) when ,ZGC It still has high performance

• ZGC rely on NUMA-aware( Unbalanced memory access ), We need our memory to support this feature

ZGC Heap memory layout

• And G1 equally ,ZGC Also based on Region The heap memory layout of

• ZGC Of Region It's dynamic

• Dynamic creation and destruction

• Dynamic Region Capacity size
  
  Size classification :

• small Region(Small Region)： Fixed size 2MB, Store less than 256KB The little object of

• medium Region(Medium Region)： Fixed size 32MB, Store more than 256KB Less than 4MB The object of

• large Region(Large Region)： Size is not fixed , Can change dynamically , But it has to be 2MB Integer multiple , Used to put larger than 4MB Large objects of , Each large Region Only one big object , So the actual capacity may be smaller than medium Region, Minimum to 4MB. large Region stay ZGC There is no reallocation in the implementation , Because copying a large object is too expensive .

ZGC Coloring pointer Technology

Colored pointer is a technique that directly stores a small amount of extra information on the pointer . Currently in Linux Next 64 Bit in the operating system 18 Bits are not addressable , But the rest 46 To support 64T Space , So far we have barely used so much memory . therefore ZGC take 46 High in the middle 4 Bit out , Used to store 4 Flag bits , remainder 42 Bits can support 4TB(2 Of 42 The next power ) Of memory , It also directly leads to ZGC No more memory can be managed than 4TB, As shown in the figure ：

• Marked0/marked1: Determine whether the object is marked
• Remapped: Determine if the application has pointed to a new address
• Finalizable: Determine whether an object can only be Finalizer visit

These are a few bits Different states represent different colors of the reference

The process of object marking is to make a three color mark , These tags are essentially only about object references , It has nothing to do with the object itself . Only the reference relation of an object determines its survival .

ZGC Memory multi mapping used （Multi-Mapping） Map multiple virtual memory addresses to the same physical memory address , This is a many to one mapping . Because stained pointers only redefine a few bits of some pointers in memory ,OS And does not support ,OS Only treat the whole pointer as a memory address , It's just that it thinks about it , To solve this problem , Using the virtual memory mapping technology of modern processors .

ZGC Reading barrier technology

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 . It should be noted that , only “ Read object references from the heap ” This code will trigger .

ZGC Recycling process

We will disassemble the above figure to understand

• Initial marker (STW)

Stop the user thread , Mark GC Root object . 1、2、4 Marked as a living object .

• Concurrent Tags

Concurrent recursion from GC Root Start traversing reachable objects ,5、8 Marked as a living object .

• move objects

Compared with 3、6、7 It's an expired object , Gray in the middle Region Need to be cleaned up and compressed , So will 4、5、8 Move to the right empty Region, There's a forward table Record this transformation .

• Fixed pointer

because 4、5、8 There was a movement , So it needs to be fixed .

summary

• ZGC As the next generation of garbage collectors , Excellent performance .ZGC The garbage collection process is almost entirely concurrent , actual STW The pause time is very short , Less than 10ms. This is due to the colored pointer and read barrier technology used .
• ZGC No “ Silver bullet ”, It needs to be tuned according to the specific characteristics of the service , And need support NUMA-aware( Unbalanced memory access ), We need our memory to support this feature . Therefore, it is still necessary to weigh according to the needs of the company's business and the actual situation .

Reference link

• https://tech.meituan.com/2020/08/06/new-zgc-practice-in-meituan.html
• https://www.baeldung.com/jvm-zgc-garbage-collector
• https://hub.packtpub.com/getting-started-with-z-garbage-collectorzgc-in-java-11-tutorial/