JDK7与8内存参数解析与修改

JDK内存

jdk内存实际是jvm内存，jvm有一个运行时数据区，其实就是对这一部分的大小分配。

运行时数据区通常包括这几个部分：程序计数器(Program Counter Register)、Java栈(VM Stack)、本地方法栈(Native Method Stack)、方法区(Method Area)、堆(Heap)

• Xss：每个线程的stack大小（栈）
• Xmx：JAVA HEAP的最大值、默认为物理内存的1/4
• Xms：JAVA HEAP的初始值，server端最好Xms与Xmx一样
• Xmn：JAVA HEAP young区的大小(相当于对-XX:newSize、-XX:MaxnewSize同时设置)
• XX:PermSize：设定内存的永久保存区域(JDK7)
• XX:MaxPermSize：设定最大内存的永久保存区域(JDK7)

JDK1.7调节参数与查看参数:

Administrator@MicroWin10-1535 MINGW64 ~/Desktop/test
$ javac Test.java

Administrator@MicroWin10-1535 MINGW64 ~/Desktop/test
$ java -Xms1024m -Xmx1024m -XX:PermSize=512m -XX:MaxPermSize=512m Test

C:\Users\Administrator>jps
19284 Jps
6628 NetworkServerControl
18072
22340
21860 Test

C:\Users\Administrator>jmap -heap 21860
Attaching to process ID 21860, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.80-b11

using thread-local object allocation.
Parallel GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 0
   MaxHeapFreeRatio = 100
   MaxHeapSize      = 1073741824 (1024.0MB)
   NewSize          = 1310720 (1.25MB)
   MaxNewSize       = 17592186044415 MB
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 536870912 (512.0MB)
   MaxPermSize      = 536870912 (512.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 268959744 (256.5MB)
   used     = 10758632 (10.260231018066406MB)
   free     = 258201112 (246.2397689819336MB)
   4.000090065522965% used
From Space:
   capacity = 44564480 (42.5MB)
   used     = 0 (0.0MB)
   free     = 44564480 (42.5MB)
   0.0% used
To Space:
   capacity = 44564480 (42.5MB)
   used     = 0 (0.0MB)
   free     = 44564480 (42.5MB)
   0.0% used
PS Old Generation
   capacity = 716177408 (683.0MB)
   used     = 0 (0.0MB)
   free     = 716177408 (683.0MB)
   0.0% used
PS Perm Generation
   capacity = 536870912 (512.0MB)
   used     = 2614432 (2.493316650390625MB)
   free     = 534256480 (509.5066833496094MB)
   0.48697590827941895% used

1523 interned Strings occupying 140352 bytes.

当然了：还有另外的配置，只是不常用

-XX:NewSize:新生代初始化内存的大小(注意：该值需要小于-Xms的值)。

-XX:MaxNewSize:新生代可被分配的内存的最大上限(注意：该值需要小于-Xmx的值)。

-Xmn:对-XX:newSize、-XX:MaxnewSize两个参数同时进行配置（注意：JDK1.4之后才有该参数）。

例如:

java -Xms1024m -Xmx1024m -XX:NewSize=512m -XX:MaxNewSize=512m -XX:OldSize=512m -XX:PermSize=512m -XX:MaxPermSize=512m Test

查看内存:

或者:

java -Xms1024m -Xmx1024m -Xmn512m -XX:PermSize=512m -XX:MaxPermSize=512m Test

结果:

在JDK1.8中，取消了PermGen，取而代之的是Metaspace，所以PermSize和MaxPermSize参数失效，取而代之的是元空间:

　例如下面配置设置元空间的大小和最大大小:(等同于JDK7的永久区)

liqiang@root MINGW64 ~/Desktop/新建文件夹 (2)
$ javac Test.java

liqiang@root MINGW64 ~/Desktop/新建文件夹 (2)
$ java -Xms1024m -Xmx1024m -XX:MetaspaceSize=512m -XX:MaxMetaspaceSize=512m Test 

查看参数:

C:\Users\liqiang>jmap -heap 2381712
Attaching to process ID 2381712, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.121-b13

using thread-local object allocation.
Parallel GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 1073741824 (1024.0MB)
   NewSize                  = 357564416 (341.0MB)
   MaxNewSize               = 357564416 (341.0MB)
   OldSize                  = 716177408 (683.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 536870912 (512.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 536870912 (512.0MB)
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 268435456 (256.0MB)
   used     = 10737448 (10.240028381347656MB)
   free     = 257698008 (245.75997161865234MB)
   4.000011086463928% used
From Space:
   capacity = 44564480 (42.5MB)
   used     = 0 (0.0MB)
   free     = 44564480 (42.5MB)
   0.0% used
To Space:
   capacity = 44564480 (42.5MB)
   used     = 0 (0.0MB)
   free     = 44564480 (42.5MB)
   0.0% used
PS Old Generation
   capacity = 716177408 (683.0MB)
   used     = 0 (0.0MB)
   free     = 716177408 (683.0MB)
   0.0% used

1597 interned Strings occupying 146680 bytes.

补充:查看JVM参数的方法

第一种:jps+jmap

C:\Users\Administrator>jps
 Main
 Jps

 Server
 NetworkServerControl

C:\Users\Administrator>jmap -heap
Attaching to process ID , please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.121-b13

using thread-local object allocation.
Parallel GC with  thread(s)

Heap Configuration:
   MinHeapFreeRatio         =
   MaxHeapFreeRatio         =
   MaxHeapSize              =  (.0MB)
   NewSize                  =  (.0MB)
   MaxNewSize               =  (.0MB)
   OldSize                  =  (.0MB)
   NewRatio                 =
   SurvivorRatio            =
   MetaspaceSize            = 536870912 (512.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 536870912 (512.0MB)
   G1HeapRegionSize         =  (.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity =  (.0MB)
   used     =  (.32839965820312MB)
   free     =  (.6716003417969MB)
   24.577509661129014% used
From Space:
   capacity =  (.5MB)
   used     =  (.48738098144531MB)
   free     =  (.0126190185546875MB)
   99.98524091397113% used
To Space:
   capacity =  (.5MB)
   used     =  (.0MB)
   free     =  (.5MB)
   0.0% used
PS Old Generation
   capacity =  (.0MB)
   used     =  (.98236846923828MB)
   free     =  (.0176315307617MB)
   5.250355103518475% used

 interned Strings occupying  bytes.

第二种:用JDK自带的工具 jconsole或者jvisualvm:

补充：32位windows平台对每个进程能管理的内存是有限制的，限制是2GB