GC 回收参数问题

我在我的 eclipse 中进行开发进行 JVM 参数测试，但是打印出来的 gc 信息和我预想(从书上看)的不一样,找了很多答案也不知道为什么，求大神指点呀！

public class OutOfMemoryTest {
	@Test
	public void heapOutOfMemory() throws InterruptedException {
		//-Xmx4M -Xms4M -Xmn2M -XX:+PrintGCDetails
		//Thread.sleep(40000);
		byte[] b = new byte[2*1024*1024];
	}
}

按照道理来说我设置堆得大小为 4M ，且不可扩展， young 区的大小为 2M ，那么 old 区的大小也应该为 2M,但是为什么 eclipse 中打印出来的 gc 信息是这样的？

Heap
 PSYoungGen      total 1536K, used 745K [0x00000000ffe00000, 0x0000000100000000, 0x0000000100000000)
  eden space 1024K, 25% used [0x00000000ffe00000,0x00000000ffe40628,0x00000000fff00000)
  from space 512K, 95% used [0x00000000fff80000,0x00000000ffffa020,0x0000000100000000)
  to   space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
 ParOldGen       total 4608K, used 2993K [0x00000000ff800000, 0x00000000ffc80000, 0x00000000ffe00000)
  object space 4608K, 64% used [0x00000000ff800000,0x00000000ffaec558,0x00000000ffc80000)
 PSPermGen       total 262144K, used 5380K [0x00000000ef800000, 0x00000000ff800000, 0x00000000ff800000)
  object space 262144K, 2% used [0x00000000ef800000,0x00000000efd41350,0x00000000ff800000)

我用 jmap 打印出来的结果也是这样， old 区的大小已经大于我整个堆得大小了？然后我将代码修改成了这样

@Test
	public void heapOutOfMemory() throws InterruptedException {
		//-Xmx64M -Xms64M -Xmn32M -XX:+PrintGCDetails
		byte[] b = new byte[32*1024*1024];
	}

打印出来的信息就正确了， old 区的大小就是 32M 了。打印结果如下:

Heap
 PSYoungGen      total 28672K, used 1966K [0x00000000fe000000, 0x0000000100000000, 0x0000000100000000)
  eden space 24576K, 8% used [0x00000000fe000000,0x00000000fe1eb9c0,0x00000000ff800000)
  from space 4096K, 0% used [0x00000000ff800000,0x00000000ff800000,0x00000000ffc00000)
  to   space 4096K, 0% used [0x00000000ffc00000,0x00000000ffc00000,0x0000000100000000)
 ParOldGen       total 32768K, used 1292K [0x00000000fc000000, 0x00000000fe000000, 0x00000000fe000000)
  object space 32768K, 3% used [0x00000000fc000000,0x00000000fc1432e8,0x00000000fe000000)
 PSPermGen       total 262144K, used 5421K [0x00000000ec000000, 0x00000000fc000000, 0x00000000fc000000)
  object space 262144K, 2% used [0x00000000ec000000,0x00000000ec54b678,0x00000000fc000000)

但是我还是有一个疑问:为什么 eden:from 的比例不是 8 ： 1. 我看了 jvm 虚拟机规范没有发现哪里有对这种情况的说明,是我做的方法不对，理解不对还是怎样的，求大家帮忙解答下。

SoloCompany

2016-10-22 00:05:48 +08:00

其实也不用去确认，你没有看到 OutOfMemoryError 就足够说明问题了，配置根本就被忽略了
我在 java8 下执行的结果并不存在你说的现象

java -Xmx4M -Xms4M -Xmn2M -XX:+PrintGCDetails -cp /private/var/folders/vv/ff28c0z560n407mjb5_x4c9h0000gn/T/CodeRunner Untitled
[GC (Allocation Failure) [PSYoungGen: 672K->496K(1536K)] 672K->512K(3584K), 0.0022537 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
[GC (Allocation Failure) [PSYoungGen: 496K->480K(1536K)] 512K->496K(3584K), 0.0007598 secs] [Times: user=0.01 sys=0.01, real=0.00 secs]
[Full GC (Allocation Failure) [PSYoungGen: 480K->0K(1536K)] [ParOldGen: 16K->428K(2048K)] 496K->428K(3584K), [Metaspace: 2727K->2727K(1056768K)], 0.0062196 secs] [Times: user=0.01 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 0K->0K(1536K)] 428K->428K(3584K), 0.0005307 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (Allocation Failure) [PSYoungGen: 0K->0K(1536K)] [ParOldGen: 428K->417K(2048K)] 428K->417K(3584K), [Metaspace: 2727K->2727K(1056768K)], 0.0042598 secs] [Times: user=0.01 sys=0.00, real=0.01 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at Untitled.main(Untitled 3.java:3)
Heap
PSYoungGen total 1536K, used 30K [0x00000007bfe00000, 0x00000007c0000000, 0x00000007c0000000)
eden space 1024K, 2% used [0x00000007bfe00000,0x00000007bfe07890,0x00000007bff00000)
from space 512K, 0% used [0x00000007bff00000,0x00000007bff00000,0x00000007bff80000)
to space 512K, 0% used [0x00000007bff80000,0x00000007bff80000,0x00000007c0000000)
ParOldGen total 2048K, used 417K [0x00000007bfc00000, 0x00000007bfe00000, 0x00000007bfe00000)
object space 2048K, 20% used [0x00000007bfc00000,0x00000007bfc685d0,0x00000007bfe00000)
Metaspace used 2759K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 300K, capacity 386K, committed 512K, reserved 1048576K

thinkmore

2016-10-22 22:12:47 +08:00

@SoloCompany 我用 jmap 打印出来确实是调整了，但是为什么会被动态调整呢？

今天我查阅了 oracle 的官方资料，上面只说了 Parallel Scavenge(64 位系统 server 模式下的默认收集器)因为默认开启了 UseAdaptiveSizePolicy 参数，所以才会对新生代的大小进行动态调整，但是为啥我使用其他收集器测试正如 jmap 中打印出来的参数那样 maxHeapSize 被修改成了 8M,我设置的 4M 没起作用。我使用这样的参数进行模拟：
```
//-Xmx6M -Xms6M -XX:+PrintGCDetails -XX:-UseAdaptiveSizePolicy -XX:+UseParNewGC
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 8388608 (8.0MB)
NewSize = 1310720 (1.25MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 21757952 (20.75MB)
MaxPermSize = 85983232 (82.0MB)
G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
capacity = 1900544 (1.8125MB)
used = 693800 (0.6616592407226562MB)
free = 1206744 (1.1508407592773438MB)
36.50533741918103% used
Eden Space:
capacity = 1703936 (1.625MB)
used = 497192 (0.47415924072265625MB)
free = 1206744 (1.1508407592773438MB)
29.179030198317307% used
From Space:
capacity = 196608 (0.1875MB)
used = 196608 (0.1875MB)
free = 0 (0.0MB)
100.0% used
To Space:
capacity = 196608 (0.1875MB)
used = 0 (0.0MB)
free = 196608 (0.1875MB)
0.0% used
tenured generation:
capacity = 4194304 (4.0MB)
used = 1415024 (1.3494720458984375MB)
free = 2779280 (2.6505279541015625MB)
33.73680114746094% used
Perm Generation:
capacity = 21757952 (20.75MB)
used = 5838448 (5.5679779052734375MB)
free = 15919504 (15.182022094726562MB)
26.833628459149097% used

```

而且 eden:from 的比例也不是 8:1 呀！

参考文档:http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html

哥，如果可以的话，加个 q 吧。 353003874