JVM garbage collector details

Serial and parallel collectors

Serial recovery : JDK1.5 The default algorithm before The disadvantage is that there is only one thread , It takes a long time for the program to stop when garbage collection is performed

Parallel recycling : Multiple threads perform garbage collection for throughput systems , The system will stop when it is recycled

serial The collector

The serial collector is the oldest , The most stable and efficient collector , There may be a longer pause , Use only one thread to recycle . The new generation 、 The old days used serial recycling ; New generation replication algorithms 、 Old age mark - Compress ; In the process of garbage collection Stop The World（ Service suspension ）

A single thread collector , When it comes to garbage collection , All other worker threads must be suspended until the end of its collection .

characteristic ：CPU The highest utilization rate , Pause time means that the user waits for a long time .

Applicable scenario ： Small applications

adopt JVM Parameters -XX:+UseSerialGC You can use a serial garbage collector .

 

ParNew The collector

ParNew The collector is essentially Serial Multithreaded version of collector . The new generation is parallel , Old age serial ; New generation replication algorithms 、 Old age mark - Compress

Parameter control ：-XX:+UseParNewGC  ParNew The collector

-XX:ParallelGCThreads Limit the number of threads

parallel The collector

Parallel Scavenge Collector similar ParNew The collector ,Parallel The collector pays more attention to the throughput of the system . The adaptive adjustment strategy can be turned on by parameters , The virtual machine collects performance monitoring information based on the current system performance , Dynamically adjust these parameters to provide the most appropriate pause time or maximum throughput ; It can also be controlled by parameters GC Time is no more than milliseconds or proportion ; New generation replication algorithms 、 Old age mark - Compress

Use multithreading to scan and compress the heap
characteristic ： Short pause time , High recovery efficiency , High throughput requirements .
Applicable scenario ： Large applications , Scientific Computing , Large scale data collection, etc .
adopt JVM Parameters XX:+USeParNewGC Turn on the concurrent tag scan garbage collector .

cms The collector

CMS（Concurrent Mark Sweep） A collector is one that aims to obtain the shortest recovery pause time . At present, a large part of Java Applications are focused on Internet sites or B/S On the server side of the system , Such applications pay special attention to the response speed of services , I hope the system has the shortest pause time , To give users a better experience .

From the name （ contain “Mark Sweep”） As you can see from the above CMS The collector is based on “ Mark - eliminate ” algorithmic , Its operation process is more complicated than the previous collectors , The whole process is divided into 4 A step , Include ：

Initial marker （CMS initial mark）

Concurrent Tags （CMS concurrent mark）

Re label （CMS remark）

Concurrent elimination （CMS concurrent sweep）

Where the initial mark 、 Re marking these two steps still requires “Stop The World”. The initial mark is just a mark GC Roots Objects that can be directly related to , fast , The concurrency tagging phase is going on GC Roots Tracing The process of , The retag phase is to fix the concurrent tagging period , The tag record of the part of the object whose tag changes as the user program continues to operate , The pause time of this stage is generally a little longer than that of the initial marking stage , But it takes much less time than concurrent tagging .

   Due to the most time-consuming concurrent marking and concurrent clearing process in the whole process , Collector threads can work with user threads , So in general ,CMS The collector's memory recovery process is performed concurrently with the user thread . Old age collector （ The new generation uses ParNew）

  advantage : Concurrent collection 、 Low pause

shortcoming ： Generate a lot of space debris 、 The concurrency phase reduces throughput

 

use “ Mark - eliminate ” Algorithm implementation , Use a multithreaded algorithm to scan the heap , Recycle objects found to be unused .

（1） Initial marker

（2） Concurrent Tags

（3） Concurrent preprocessing

（4） Re label

（5） Concurrent elimination

（6） Concurrent reset

characteristic ： Response time first , Reduce garbage collection pause time

Adaptive scene ： Large servers, etc .

adopt JVM Parameters -XX:+UseConcMarkSweepGC Set up

g1 The collector

stay G1 in , The heap is divided into Many consecutive areas (region). use G1 Algorithm for recycling , Absorbed CMS Collector features .

characteristic ： Support a large pile , High throughput

  -- Support more CPU And the garbage collection thread

  -- When the main thread is paused , Use parallel collection

  -- With the main thread running , Use concurrent collection

Real time target ： Configurable in N In milliseconds, it only takes up M Millisecond time for garbage collection

adopt JVM Parameters -XX:+UseG1GC Use G1 Garbage collector

Be careful : Concurrency refers to a processor processing multiple tasks at the same time . 
Parallel refers to multiple processors or multi-core processors processing multiple different tasks at the same time . 
Concurrency is logically simultaneous （simultaneous）, And parallel is physical simultaneity . 
A metaphor ： Concurrent is a person eat three steamed bread at the same time , In parallel, three people eat three steamed buns at the same time .