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One 、 Concept

CMS：Concurrent Mark Sweep
CMS A collector is a way to get Minimum recovery pause time Collector for target .

Two 、 Algorithm

Mark - eliminate

3、 ... and 、 The process （ step ）

Because in the whole process , Concurrent tagging and concurrent cleanup , Collector threads can work with user threads , So in general ,CMS The collector's memory recovery process is executed concurrently with the user thread .

1、 Initial marker CMS initial mark 【STW】

Mark GC Roots Directly associate objects , no need Tracing, fast , yes STW Of

2、 Concurrent Tags CMS concurrent mark【 Concurrent 】

Conduct GC Roots Tracing , Find the remaining objects in the reference chain , It's concurrent

3、 Re label CMS remark 【STW】

Modify the content of concurrent tags due to user program changes , Relabel incremental garbage , yes STW Of

4、 Concurrent elimination CMS concurrent sweep 【 Concurrent 】

Clear unreachable objects to reclaim space , At the same time, new garbage is generated , Save it for the next clean-up called floating trash

Four 、 excellent vacancy

advantage ： Concurrent collection 、 Low pause
shortcoming ： Generate a lot of space debris 、 The concurrent phase reduces throughput

5、 ... and 、 Be careful

1、 Whether the initial tag is single threaded or multi-threaded ？

answer ： according to JDK edition
<=JDK7, The initial tag defaults to Single thread （ Multithreading incurs additional threading overhead ）
>=JDK8, The initial tag defaults to Multithreading （ The system thinks it's multithreaded code ）

2、 Retag whether it's single threaded or multi-threaded ？

answer ： Re marking is Multithreading
reason ： Multithreading has been created during concurrent marking , There is no additional thread overhead for re tagging , Use multithreading directly

3、 Improve efficiency by setting the initial flag

Premise ：JDK8,CMS, The business is single threaded （ The application is single threaded ）
adjustment ： Set the initial flag to single threaded , Improve garbage collection efficiency