JDK8堆默认比例

JDK8

默认的，新生代 ( Young ) 与老年代 ( Old ) 的比例的值为 1:2 ( 该值可以通过参数 –XX:NewRatio 来指定 )，即：新生代 ( Young ) = 1/3 的堆空间大小。

老年代 ( Old ) = 2/3 的堆空间大小。其中，新生代 ( Young ) 被细分为 Eden 和 两个 Survivor 区域，这两个 Survivor 区域分别被命名为 from 和 to，以示区分。

默认的，Edem : from : to = 8 : 1 : 1 ( 可以通过参数 –XX:SurvivorRatio 来设定 )，即： Eden = 8/10 的新生代空间大小，from = to = 1/10 的新生代空间大小。

‍JVM 每次只会使用 Eden 和其中的一块 Survivor 区域来为对象服务，所以无论什么时候，总是有一块 Survivor 区域是空闲着的。

因此，新生代实际可用的内存空间为 9/10 ( 即90% )的新生代空间。

Java 中的堆是 JVM 所管理的最大的一块内存空间，主要用于存放各种类的实例对象。

在 Java 中，堆被划分成两个不同的区域：新生代 ( Young )、老年代 ( Old )。新生代 ( Young ) 又被划分为三个区域：Eden、From Survivor、To Survivor。

这样划分的目的是为了使 JVM 能够更好的管理堆内存中的对象，包括内存的分配以及回收。

堆的内存模型大致为：

JDK8中的默认参数：（windows平台）

见链接：http://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html#BABHDABI

http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html

参数的默认值：

ratio(比，比率;比例;系数)

-XX:NewRatio

-XX:NewRatio=ratio

Sets the ratio between young and old generation sizes. By default, this option is set to 2.The following example shows how to set the young/old ratio to 1:

-XX:NewRatio=1

-XX:SurvivorRatio

-XX:SurvivorRatio=ratio

Sets the ratio between eden space size and survivor space size. By default, this option is set to 8. The following example shows how to set the eden/survivor space ratio to 4:

-XX:SurvivorRatio=4

-Xmssize

Sets the initial size (in bytes) of the heap. This value must be a multiple of 1024 and greater than 1 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes.

The following examples show how to set the size of allocated memory to 6 MB using various units:

-Xms6291456

-Xms6144k

-Xms6m

If you do not set this option, then the initial size will be set as the sum of the sizes allocated for the old generation and the young generation. The initial size of the heap for the young generation can be set using the -Xmn option or the -XX:NewSize option.

-Xmxsize

Specifies the maximum size (in bytes) of the memory allocation pool in bytes. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -Xms and -Xmx are often set to the same value. For more information, see Garbage Collector Ergonomics at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gc-ergonomics.html

The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-Xmx83886080

-Xmx81920k

-Xmx80m

The -Xmx option is equivalent to -XX:MaxHeapSize.

-XX:MaxHeapSize=size

Sets the maximum size (in byes) of the memory allocation pool. This value must be a multiple of 1024 and greater than 2 MB. Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate gigabytes. The default value is chosen at runtime based on system configuration. For server deployments, -XX:InitialHeapSize and -XX:MaxHeapSize are often set to the same value. For more information, see Garbage Collector Ergonomics at http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gc-ergonomics.html

The following examples show how to set the maximum allowed size of allocated memory to 80 MB using various units:

-XX:MaxHeapSize=83886080

-XX:MaxHeapSize=81920k

-XX:MaxHeapSize=80m

The -XX:MaxHeapSize option is equivalent to -Xmx.

如下图所示，

如下图所示，