Learn JVM garbage collection 05 - root node enumeration, security points, and security zones (hotspot)

Catalog

Root node enumeration

What is root node enumeration ？

How it works

safer

selection

Thread the interrupt

Active

Preemptive （ Almost no use ）

The safety area

What is a safe area ？

How to solve the problem

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Root node enumeration

What is root node enumeration ？

         Judge the life and death of the object according to the method of reachability analysis , We need to find every starting point . This search process is called root node enumeration .

How it works

         up to now , All garbage collector root node enumeration operations must pause the user thread . In other words, it will face similar problems to memory defragmentation “Stop The World” .

        Virtual machines have a way to directly get where object references exist . stay HotSpot in , Through a group called OopMap The implementation of the data structure of . Information about references will be recorded here .

safer

OopMap Will make information records at specific locations , These locations are called safety points .

The emergence of safety points , It also gives the thread a proper time to stop , When the program is executed , To execute above the safe point, stop .

selection

If there are many safety points , that OopMap It will be great. , The memory overhead becomes larger .

Few safety points , The waiting time of the garbage collector will be very long .

The selection of safety points requires a standard ： With method calls 、 Loop jump 、 Abnormal jump, etc. , Instruction sequence reuse The instructions of the function will produce a safety point .

Why? ？

         The selection of safe point location is basically based on “ Whether the program has the characteristics of long execution ” As the standard Make the selected , Because the execution time of every instruction is very short , The program is unlikely to be executed for a long time due to the length of instruction stream “ Long execution ” The most obvious feature of instruction sequence is the reuse of instruction sequence , For example, method calls 、 Loop jump 、 Abnormal jump All belong to instruction sequence reuse , So only instructions with these functions can produce safe points .

Thread the interrupt

Here the so-called thread interrupt , It means that the implementation makes the thread run to the nearest safe point and then stop . There are two ways to achieve

Active interrupt 、 Preemptive interrupt

Active

In a safe place , Set up a Polling flag bit  , When interruptions are needed , Set to true .

When a thread executes an instruction to a safe point , The polling flag bit will be checked . If it's true , Then the thread suspends itself , Implementation interrupt .

besides , It also needs to be set on the instructions that need to allocate memory Polling flag bit . Ensure that garbage collection is performed before allocating memory . 

Preemptive （ Almost no use ）

This is a kind of almost useless .

         When the garbage collector raises an interrupt , Stop all threads , See which thread is not on the safe point , Wake up it . Let it run to safety .

The safety area

         When the thread is not running （ such as Sleep The state or Blocked state ） when , Can't go to a safe place to hang up . The safe area is to solve this matter .

What is a safe area ？

        Refers to in a piece of program code , There are no quoted relationship changes , That is, it is safe to interrupt threads anywhere in this area .

How to solve the problem

        When a thread enters a safe zone , Will mark themselves in a safe area . At this point, virtual machines don't have to worry about them during garbage collection .

        When you go out, you will ask whether the virtual machine has completed the root node enumeration （ Or other stages in the garbage collection process that need to pause the user thread ）. If it's done , The thread continues to execute . If not completed , The program to stop , Wait for instructions to leave the safe area .

 

That's all for the canto .

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