JVM G1垃圾回收器总结

G1垃圾收集器

G1是一个面向服务端的JVM垃圾收集器，适用于多核处理器、大内存容量的服务端系统。 它满足短时间停顿的同时达到一个高的吞吐量。从JDK 9开始，G1成为默认的垃圾回收器。
当应用有以下任何一种特性时非常适合用G1：

• Full GC持续时间太长或者太频繁
• 对象的创建速率和存活率变动很大
• 应用不希望停顿时间长(长于0.5s甚至1s)

G1对Heap的划分

G1按固定大小把内存划分为很多小区域(region)，这个堆大概有2000多块；在逻辑上，某些小区域构成Eden，某些构成Survivor，某些构成老年代，这些小区域物理上是不相连的，并且构成新生代和老年代的区域可以动态改变。
可以通过命令行参数 -XX:NewRatio=n来配置老年代和新生代的比例，默认为2，即比例为2:1； -XX:SurvivorRatio=n则可以配置Eden与Survivor的比例，默认为8。

Young GC

当Eden区的空间占满之后，会触发Young GC，G1将Eden和Survivor中存活的对象拷贝到Survivor，或者直接晋升到Old Region中。Young GC的执行是多线程并发的，期间会停顿所有的用户线程（STW）。

Old GC

当堆空间的占用率达到一定阈值后会触发Old GC（阈值由命令参数-XX:InitiatingHeapOccupancyPercent设定，默认值45），Old GC分五个步骤完成：

初始标记（Initial Mark，STW）

对Survivor区域扫描，标记出可能拥有老年代对象引用的根区域（Root Region），该阶段附属于一次Young GC的最后阶段，所以是STW。在GC日志中会被记录为GC pause (young)(inital-mark)

根区域扫描（Root Region Scan）

从上一阶段标记的根区域中，标记所有拥有老年代对象引用的存活对象，这是一个并发的过程，而且必须在进行下一次Young GC之前完成

并发标记（Concurrent Marking）

从上一阶段标记的存活对象开始，并发地跟踪所有可达的老年代对象，期间可以被Young GC打断

最终标记（Remark，STW）

G1并行地跟踪在上面阶段经过更新的存活对象，找到未被标记的存活的对象，这是一个STW的阶段，会用到一个初始快照（SATB）算法。在此期间完全空闲的区域会被直接重置回收。

复制/清除（Copying/Cleanup，STW）

• G1统计存活对象和完全空闲的区域，完全空闲区域将被重置回收
• 执行清理RSet的操作
• 将存活对象复制到新的未被占用的区域。执行这一步时可以只对新生代操作，这时G1将其记录为[GC pause (young)]；也可对新生代和一部分老年代都进行处理，这时被记录为[GC Pause (mixed)]，这些老年代要根据其“存活度”去选择。
  
  

G1相关的命令

java命令行参数

Option	Description
-XX:+UseG1GC	Use the Garbage First (G1) Collector
-XX:MaxGCPauseMillis=n	Sets a target for the maximum GC pause time. This is a soft goal, and the JVM will make its best effort to achieve it.
-XX:InitiatingHeapOccupancyPercent=n	Percentage of the (entire) heap occupancy to start a concurrent GC cycle. It is used by GCs that trigger a concurrent GC cycle based on the occupancy of the entire heap, not just one of the generations (e.g., G1). A value of 0 denotes 'do constant GC cycles'. The default value is 45.
-XX:NewRatio=n	Ratio of old/newgeneration sizes. The default value is 2.
-XX:SurvivorRatio=n	Ratio of eden/survivor space size. The default value is 8.
-XX:MaxTenuringThreshold=n	Maximum value for tenuring threshold. The default value is 15.
-XX:ParallelGCThreads=n	Sets the number of threads used during parallel phases of the garbage collectors. The default value varies with the platform on which the JVM is running.
-XX:ConcGCThreads=n	Number of threads concurrent garbage collectors will use. The default value varies with the platform on which the JVM is running.
-XX:G1ReservePercent=n	Sets the amount of heap that is reserved as a false ceiling to reduce the possibility of promotion failure. The default value is 10.
-XX:G1HeapRegionSize=n	With G1 the Java heap is subdivided into uniformly sized regions. This sets the size of the individual sub-divisions. The default value of this parameter is determined ergonomically based upon heap size. The minimum value is 1Mb and the maximum value is 32Mb.

查看JVM默认的垃圾回收器

java -XX:+PrintCommandLineFlags -version

如

-XX:G1ConcRefinementThreads=4 -XX:InitialHeapSize=134217728 -XX:MaxHeapSize=2147483648 -XX:+PrintCommandLineFlags -XX:ReservedCodeCacheSize=251658240 -XX:+SegmentedCodeCache -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseG1GC 
java version "9.0.4"
Java(TM) SE Runtime Environment (build 9.0.4+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.4+11, mixed mode)

查看JVM当前堆情况

jmap -heap pid 

如

jmap -heap  20932
Attaching to process ID 20932, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.131-b11

using thread-local object allocation.
Garbage-First (G1) GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 4164943872 (3972.0MB)
   NewSize                  = 1363144 (1.2999954223632812MB)
   MaxNewSize               = 2498756608 (2383.0MB)
   OldSize                  = 5452592 (5.1999969482421875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 1048576 (1.0MB)

Heap Usage:
G1 Heap:
   regions  = 3972
   capacity = 4164943872 (3972.0MB)
   used     = 2132264664 (2033.4860458374023MB)
   free     = 2032679208 (1938.5139541625977MB)
   51.19551978442604% used
G1 Young Generation:
Eden Space:
   regions  = 829
   capacity = 2317352960 (2210.0MB)
   used     = 869269504 (829.0MB)
   free     = 1448083456 (1381.0MB)
   37.51131221719457% used
Survivor Space:
   regions  = 9
   capacity = 9437184 (9.0MB)
   used     = 9437184 (9.0MB)
   free     = 0 (0.0MB)
   100.0% used
G1 Old Generation:
   regions  = 1254
   capacity = 1741684736 (1661.0MB)
   used     = 1253557976 (1195.4860458374023MB)
   free     = 488126760 (465.51395416259766MB)
   71.9738739215775% used

19747 interned Strings occupying 1885184 bytes.

参考资料

• Getting Started with the G1 Garbage Collector
• JVM垃圾收集器简介