JVM garbage collector

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Preface

Last time JVM Garbage collection algorithm , After the foreshadowing of the previous article, this article focuses on JVM Of GC Recyclers , The main contents are referred to 《 In depth understanding of Java virtual machine 》.
This article focuses on CMS and G1GC, Other recyclers will be directly carried by .

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One 、 Common garbage collectors

Here are JDK8-JDK9 Common garbage collector combinations ：

From the above figure, we can draw a conclusion ：

1. JDK8 Supported garbage collection combinations are ：Serial+Serial Old,Parallel Scavenge+Parallel Old,CMS+ParNew,Parallel Scavenge+Serial Old,G1 Five combinations ;
2. JDK9 stay JDK8 On the basis of the new Serial + CMS,ParNew+Serial Old Two combinations .

among JDK8 default GC The combination is Parallel Scavenge+Parallel Old,JDK9 Default GC yes G1GC.

Two 、CMS The collector

1. brief introduction

CMS（concurrent mark sweep） Collector is a kind of collector that aims to obtain the shortest recovery pause time .
From its name （mark sweep） You can see that it is based on tags - Clear algorithm implementation .

2. Collection process

The process is relatively complicated , There are mainly 4 part ：

1. Initial marker , Just mark it GC Roots Objects that can be directly related , Short time , Meeting STW;
2. Concurrent Tags , from GC Roots The process of traversing the whole object graph begins with the directly related object , Time consuming . Execute with user thread , There is no need to pause .
3. Re label , Increment mark , Resolve the objects that cause tag changes during user thread runtime during concurrent tagging , Less time consuming , Meeting STW（ The solution is the recyclable garbage added by the user thread during concurrent marking ）;
4. Concurrent elimination , Objects that are judged to be dead in the clear and delete mark stage . Because it is executed together with the user thread , There is no need to pause .

3. Advantages and disadvantages

advantage ：
Concurrent collection , Low pause .

shortcoming ：

1. Because it is a concurrent collection, it is very sensitive to processor resources , Will take up part of CPU;
2. Based on tags - Clear algorithm implementation , There is a memory fragmentation problem （ Mark - Clear algorithm problems ）;
3. Can't handle floating garbage , User threads are running in both the concurrent marking and concurrent cleanup phases , The garbage generated during this period can only be left for the next collection , This is floating garbage .

3、 ... and 、G1 The collector

1. brief introduction

G1 Collector is a milestone in the development of garbage collector technology , It pioneers the design idea of collector for local collection and is based on region Memory form of layout （ Physical zoning , Logical generation ）.
It is worth mentioning that JDK8 Support G1GC,JDK9 Default garbage collector , And CMS Reduced to a deprecated garbage collector , As if CMS Alternative optimized version .
It is no longer measured by which generation it belongs , It's about which memory holds the most garbage , Maximum return , namely Mixed GC Pattern .

2. Collection process

There are mainly 4 A process ：

1. Initial marker , Mark GC Roots Objects that can be directly related to （ For a short time STW）;
2. Concurrent Tags , from GC Roots Start the reachability analysis of the objects in the heap , Recursively scan the whole heap to find the object to be recycled , Execute in parallel with the user thread （ There is no need to pause , Longer time consuming ）;
3. Final marker , Increment mark , Resolve the objects that cause tag changes during user thread runtime during concurrent tagging （ Shorter STW）;
4. Screening markers , Is responsible for updating Region Statistical data , To each Region Sorted by recovery value and cost , Make a recycling plan based on the user's expected pause time , You can choose any number Region Make up a recycling set , Then decide to recycle that part Region Of live objects copied to empty region in , And clean up the whole old region All the space of （ Longer STW, Core design , The expected delay time can be set . The default is 200ms）.

3. Advantages and disadvantages

advantage ：
1. Use alone , There's no need to combine , Less parameters , Low tuning cost ;
2. You can set the estimated pause time ;
3. There will be no memory fragmentation （ A collector based on tag collation Algorithm ）.

shortcoming ：
1. Need more memory support （ branch region）;
2. Yes CPU Multi core is required （ Many concurrent operations ）.

summary

In large memory (>=6G) Under the system, if you choose to use CMS and G1 Recommended G1; On the contrary, the system has small memory （<6G） Because of G1GC Partitions occupy more space , In theory, it will be better than CMS Bad .