Learn the garbage collector of JVM -- a brief introduction to Shenandoah collector

There are mainly two low latency garbage collectors ：

Shenandoah The collector and Officially launched ZGC The collector

Shenandoah

Brief introduction

Shenandoah It is not officially launched , But compared to ZGC The collector is more like G1 The next generation of collectors .

First , The adoption is based on Region The heap memory layout of .

secondly , Have the ability to store magnified objects Humongous Region.

and , Use the default recycling strategy with the greatest value of processing recycling first .

Then with G1 The difference between ？

1. Support concurrent collation algorithm
2. Default generation free
3. Use a global data structure called a connection matrix to record cross Region Reference relation of .

There are no generations , It is out of consideration of cost performance .

The connection matrix can be regarded as a two-dimensional table , If Region n There is an object pointing to Region m, It's on the form n That's ok m Put a mark in the column . Judge by such a mark Region Reference relationship between .

working process

1.         Initial marker ： And G1 equally , First mark with GC Roots Directly associated objects , This stage is still yes “Stop The World” Of , But the pause time has nothing to do with the heap size , Only with GC Roots The quantity is related to .

2.         Concurrent Tags ： And G1 equally , Traversal object graph , Mark all accessible objects , This stage is together with the user thread Concurrent Of , The length of time depends on the number of surviving objects in the heap and the structural complexity of the object graph .

3.         Final marker ： And G1 equally , Deal with the rest SATB scanning , And at this stage, it is calculated that the recycling value is the highest Region, Will these Region Make up a collection . There will also be a short pause in the final marking phase .

4.         Concurrent cleanup ： This stage is used to clean up those objects that have not been found in the whole area Region.

5.         Concurrent recovery ： The concurrent recycle phase is Shenandoah Prior to HotSpot The core differences of other collectors in . At this stage ,Shenandoah You need to make a copy of the living object back in the collection to the unused one Region In . The duration of the concurrent recycle phase depends on the size of the recycle set .

6.         Initial reference update ： After copying objects in the concurrent recycle phase , You also need to fix all references to old objects in the heap to new addresses after copying , This operation is called reference update . The initialization phase of the reference update doesn't actually do anything specific , This phase is only set up to create a thread collection point , Ensure that all concurrent recovery phases are in progress All collector threads have completed the object movement task assigned to them . There will be a very short pause .

7.         Concurrent reference updates ： Actually start the reference update operation , This phase is concurrent with the user thread , The length of time depends on the number of references involved in memory . Concurrent reference updates are different from concurrent tags , it No longer need to search along the object graph , Just follow the memory physical address order , Search for reference types linearly , Change the old value to the new value .

8.         Final quote update ： After resolving the reference update in the heap , And fix the existing GC Roots Citation in . This stage is Shenandoah The last pause of , The pause time is only with GC Roots The quantity is related to .

9.         Concurrent cleanup ： After concurrent recycling and reference updates , The whole collection of all Region There are no more living objects , Finally, call the concurrent cleanup process again to recycle these Region Of memory space , For later allocation of new objects .