ZGC concurrent identity and multi view address mapping in concurrent transition phase

M0,M1,Remapped The three multi view address mappings are zgc The key premise to realize concurrency identification and concurrency transfer , These three addresses actually correspond to the same physical address , Corresponding to the same underlying object , Broadly zgc The address mapping transformation relationship in the process is as follows :
The first round zgc:
1. Initial identification stage
The address views of objects in the initial identification phase are Remapped
2. Concurrency identification phase
The address of the surviving object is from Remapped Address becomes M0 Address , The address of the non surviving object remains Remapped Mapping phase , The address view of the newly allocated object in this phase is M0
3. Concurrent transition phase
The living object （ The address view is M0） Is moved to a new address space and re identified as Remapped Address view , Non living objects （ The address view is Remapped） Will be garbage collected , The address view of the newly allocated object in this phase is Remapped

The second round zgc:
1. Initial identification stage
The address views of objects in the initial identification phase are Remapped, perhaps M0,M0 The reason for the address view is that some objects have not been transferred or accessed in the previous round of concurrent transfer , Therefore, the status after the last round of concurrent identification is still maintained M0 View .– Here comes another meaning , In a round of zgc in , Although it marks the whole heap space , But not all tagged live objects are transferred , Not all garbage objects are recycled , For those pages with a small proportion of garbage , In a round of zgc The surviving objects in these pages will not be transferred , Garbage objects are not recycled .
2. Concurrency identification phase
The address of the surviving object is from Remapped/M0 Address becomes M1 Address , The address of the non surviving object remains Remapped/M0 Mapping phase , The address view of the newly allocated object in this phase is M1
3. Concurrent transition phase
The living object （ The address view is M1） Is moved to a new address space and re identified as Remapped Address view , Non living objects （ The address view is Remapped perhaps M0） Will be garbage collected , The address view of the newly allocated object in this phase is Remapped

Rear multi wheel zgc in , The object view of the concurrency identifier will be displayed in M0 and M1 Alternate between

Besides zgc Fast multi view mapping in , Each time from Remapped Mark it as M0 perhaps M1 You only need to change the number in the object address 42 Position to the first 45 Bit can complete the address mapping operation , and cms perhaps g1 To mark an object in, you need to access the object header of the specific object pointed to by the object address , And mark the flag bit in the object header , This process involves accessing object memory , and zgc No need to access object memory , This is also zgc The reason why it is faster .