Minor GC和Full GC触发条件

程序员文章站 2022-05-06 14:10:00

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一、Minor GC触发条件
1、eden区满时，触发MinorGC。即申请一个对象时，发现eden区不够用，则触发一次MinorGC。

注：新生代分为三个区域，eden space, from space, to space。默认比例是8:1:1。在MinorGC时，会把存活的对象复制到to space区域，如果to space区域不够，则利用担保机制进入老年代区域。

对eden space, from space, to space的理解：每次分配eden space空间，如果不够，则小于 to space大小的对象复制到 to space，然后to space和from space换位置，所以我们看到的to space一直是空的。

测试代码：

	private static final int _1M = 1024 * 1024;

	private static void testMinorGC() {

		/* eden space为8M，from/to space各为1M */
		LargeObject largeOb1 = new LargeObject(_1M * 1 / 2, "largeOb1");
		LargeObject largeOb2 = new LargeObject(_1M * 1, "largeOb2");
		LargeObject largeOb3 = new LargeObject(_1M * 2, "largeOb3");
		largeOb3 = null;
		LargeObject largeOb4 = new LargeObject(_1M * 3, "largeOb4");
		LargeObject largeOb5 = new LargeObject(_1M * 2, "largeOb5");
	}

	public static void main(String[] agrs) {
		testMinorGC();
	}

	static class LargeObject {
		private byte[] data;
		private String name;

		public LargeObject(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObject " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}
	}

测试环境：JDK1.8.0_144，Java HotSpot(TM) 64-Bit Server VM，默认垃圾收集器为PS Scavenge+PS MarkSweep。

-Xms30m -Xmx30m -Xmn10m -XX:+PrintGCDetails -XX:+PrintHeapAtGC

堆总大小为30m，新生代为10m，打印GC信息，在GC前后打印内存信息。在eclipse下运行参考eclipse设置运行JVM参数。

打印结果及分析：

Over Constructing LargeObject largeOb1

Over Constructing LargeObject largeOb2

Over Constructing LargeObject largeOb3

Over Constructing LargeObject largeOb4

{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 7640K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 93% used [0x00000000ff600000,0x00000000ffd760d8,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 0K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 0% used [0x00000000fe200000,0x00000000fe200000,0x00000000ff600000)
 Metaspace       used 2777K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K


// 实例化largeOb1、largeOb2、largeOb3、largeOb4共使用了eden space 6656K ，从上面信息可以看到eden space还剩余573m，初始化LargeOb5时，需要2048K。
// 由于eden space不够给新的对象分配内存，所以触发一次MinorGC。
// 7640K->1000K(9216K，eden space+from space)，新生代GC结果从7640K变成了1000K，这时新生代只存在largeOb1对应的内存，因为largeOb1对应的内存为0.5M，小于to space区大小，进入from space？不太懂。
// largeOb2和largeOb4由于对应的大于to space区大小，担保进入到老年代。
// 7640K->5232K(29696K，eden space+from space+ParOldGen)，总的内存占用变化，回收了2408K空间。
// largeOb3之前的内存没有引用（largeOb3=null），所以被回收了。
[GC (Allocation Failure) [PSYoungGen: 7640K->1000K(9216K)] 7640K->5248K(29696K), 0.0030601 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [PSYoungGen: 7640K->616K(9216K)] 7640K->5232K(29696K), 0.0031851 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 


// 回收之后的内存情况，可以看到from space存在largeOb1，ParOldGen存在largeOb2和largeOb4。
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 616K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 60% used [0x00000000ffe00000,0x00000000ffe9a020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 4616K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 22% used [0x00000000fe200000,0x00000000fe682030,0x00000000ff600000)
 Metaspace       used 2777K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb5

// 分配完largeOb5，可以看到largeOb5的内存被分配在了新生代的eden space。
Heap
 PSYoungGen      total 9216K, used 2906K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 27% used [0x00000000ff600000,0x00000000ff83ca88,0x00000000ffe00000)
  from space 1024K, 60% used [0x00000000ffe00000,0x00000000ffe9a020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 4616K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 22% used [0x00000000fe200000,0x00000000fe682030,0x00000000ff600000)
 Metaspace       used 2784K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 300K, capacity 386K, committed 512K, reserved 1048576K

2、Full GC之前调用，仅适用于Parallel Scavenge（-XX:+UseParallelGC），虚拟机运行在Server模式下的默认收集器组合。

判断一个对象是否存活，除了GC Roots引用之外，还有一个条件就是对象是否重写了finalize方法，如果对象重写了该方法，则会交给FQueue队列去执行，如果执行该方法后被重新关联，则在下次回收时不会被回收，否则下次回收，该方法只执行一次。

测试代码：

	private static final int _1M = 1024 * 1024;

	private static void testMinorGCOverriddeFinalize() {


		/* Eden区为8M，from/to space各为1M */
		LargeObjectOverrideFinalize largeOb1 = new LargeObjectOverrideFinalize(_1M * 1 / 2, "largeOb1");
		LargeObjectOverrideFinalize largeOb2 = new LargeObjectOverrideFinalize(_1M * 1, "largeOb2");
		LargeObjectOverrideFinalize largeOb3 = new LargeObjectOverrideFinalize(_1M * 2, "largeOb3");
		largeOb1 = null;
		largeOb3 = null;
		LargeObjectOverrideFinalize largeOb4 = new LargeObjectOverrideFinalize(_1M * 3, "largeOb4");
		LargeObjectOverrideFinalize largeOb5 = new LargeObjectOverrideFinalize(_1M * 2, "largeOb5");


		/* 保证调用finalize完毕 */
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}


		System.gc();
	}

	public static void main(String[] agrs) {
		testMinorGCOverriddeFinalize();
	}

	static class LargeObjectOverrideFinalize {
		private byte[] data;
		private String name;

		public LargeObjectOverrideFinalize(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObjectOverrideFinalize " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}

		@Override
		public void finalize() {
			System.out.println(
					"Finalize LargeObjectOverrideFinalize " + name + " , release " + (data.length / 1024) + "K.");
		}
	}

测试环境：JDK1.8.0_144，Java HotSpot(TM) 64-Bit Server VM，默认垃圾收集器为PS Scavenge+PS MarkSweep。

-Xms30m -Xmx30m -Xmn10m -XX:+PrintGCDetails -XX:+PrintHeapAtGC

堆总大小为30m，新生代为10m，打印GC信息，在GC前后打印内存信息。

打印结果及分析：

Over Constructing LargeObjectOverrideFinalize largeOb1

Over Constructing LargeObjectOverrideFinalize largeOb2

Over Constructing LargeObjectOverrideFinalize largeOb3

Over Constructing LargeObjectOverrideFinalize largeOb4

{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 7640K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 93% used [0x00000000ff600000,0x00000000ffd760d8,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 0K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 0% used [0x00000000fe200000,0x00000000fe200000,0x00000000ff600000)
 Metaspace       used 2774K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K

// 对比未实现finalize方法的回收情况，实现了finalize方法的对象都没有被回收
[GC (Allocation Failure) [PSYoungGen: 7640K->736K(9216K)] 7640K->7400K(29696K), 0.0047814 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 736K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 71% used [0x00000000ffe00000,0x00000000ffeb8030,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 6664K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 32% used [0x00000000fe200000,0x00000000fe882040,0x00000000ff600000)
 Metaspace       used 2774K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K
}
Finalize LargeObjectOverrideFinalize largeOb1 , release 512K.
Finalize LargeObjectOverrideFinalize largeOb3 , release 2048K.
Over Constructing LargeObjectOverrideFinalize largeOb5

{Heap before GC invocations=2 (full 0):
 PSYoungGen      total 9216K, used 3108K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 28% used [0x00000000ff600000,0x00000000ff8510f0,0x00000000ffe00000)
  from space 1024K, 71% used [0x00000000ffe00000,0x00000000ffeb8030,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 6664K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 32% used [0x00000000fe200000,0x00000000fe882040,0x00000000ff600000)
 Metaspace       used 2775K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K

// 调用System.gc()之后先进行一次MinorGC，largeOb1在from space区域，largeOb3在ParOldGen，所以这次只把largeOb5放入了老年代。
[GC (System.gc()) [PSYoungGen: 3108K->712K(9216K)] 9772K->9424K(29696K), 0.0059394 secs] [Times: user=0.05 sys=0.02, real=0.01 secs] 
Heap after GC invocations=2 (full 0):
 PSYoungGen      total 9216K, used 712K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 69% used [0x00000000fff00000,0x00000000fffb2050,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 8712K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 42% used [0x00000000fe200000,0x00000000fea82050,0x00000000ff600000)
 Metaspace       used 2775K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=3 (full 1):
 PSYoungGen      total 9216K, used 712K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 69% used [0x00000000fff00000,0x00000000fffb2050,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 8712K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 42% used [0x00000000fe200000,0x00000000fea82050,0x00000000ff600000)
 Metaspace       used 2775K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K

// Full GC把from space的largeOb1回收，老年代回收了largeOb3。
[Full GC (System.gc()) [PSYoungGen: 712K->0K(9216K)] [ParOldGen: 8712K->6696K(20480K)] 9424K->6696K(29696K), [Metaspace: 2775K->2775K(1056768K)], 0.0186298 secs] [Times: user=0.06 sys=0.00, real=0.02 secs] 
Heap after GC invocations=3 (full 1):
 PSYoungGen      total 9216K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 6696K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 32% used [0x00000000fe200000,0x00000000fe88a260,0x00000000ff600000)
 Metaspace       used 2775K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K
}
Heap
 PSYoungGen      total 9216K, used 82K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 1% used [0x00000000ff600000,0x00000000ff614920,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 6696K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 32% used [0x00000000fe200000,0x00000000fe88a260,0x00000000ff600000)
 Metaspace       used 2781K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 300K, capacity 386K, committed 512K, reserved 1048576K

注：如果逃脱垃圾回收参考使用finalize逃脱垃圾回收。

二、Full GC触发条件

1、老生代空间不够分配新的内存

测试代码：

	private static final int _1M = 1024 * 1024;

	private static void testFullGC() {

		/* Eden区为8M，from/to space各为1M */
		LargeObject largeOb1 = new LargeObject(_1M * 8, "largeOb1");
		largeOb1 = null;
		LargeObject largeOb2 = new LargeObject(_1M * 15, "largeOb2");
	}

	public static void main(String[] agrs) {
	        testFullGC();
	}

	static class LargeObject {
		private byte[] data;
		private String name;

		public LargeObject(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObject " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}
	}

测试环境：JDK1.8.0_144，Java HotSpot(TM) 64-Bit Server VM，默认垃圾收集器为PS Scavenge+PS MarkSweep。

-Xms30m -Xmx30m -Xmn10m -XX:+PrintGCDetails -XX:+PrintHeapAtGC

堆总大小为30m，新生代为10m，打印GC信息，在GC前后打印内存信息。

打印结果及分析：

注意Full GC原因，Full GC (Allocation Failure)，分配失败。

Over Constructing LargeObject largeOb1


{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 1148K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 14% used [0x00000000ff600000,0x00000000ff71f3d8,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 8192K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 40% used [0x00000000fe200000,0x00000000fea00010,0x00000000ff600000)
 Metaspace       used 2864K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
[GC (Allocation Failure) [PSYoungGen: 1148K->704K(9216K)] 9340K->8904K(29696K), 0.0008682 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 704K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 68% used [0x00000000ffe00000,0x00000000ffeb0030,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 8200K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 40% used [0x00000000fe200000,0x00000000fea02010,0x00000000ff600000)
 Metaspace       used 2864K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=2 (full 0):
 PSYoungGen      total 9216K, used 704K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 68% used [0x00000000ffe00000,0x00000000ffeb0030,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 8200K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 40% used [0x00000000fe200000,0x00000000fea02010,0x00000000ff600000)
 Metaspace       used 2864K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
[GC (Allocation Failure) [PSYoungGen: 704K->720K(9216K)] 8904K->8920K(29696K), 0.0006179 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=2 (full 0):
 PSYoungGen      total 9216K, used 720K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 70% used [0x00000000fff00000,0x00000000fffb4040,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 8200K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 40% used [0x00000000fe200000,0x00000000fea02010,0x00000000ff600000)
 Metaspace       used 2864K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}

{Heap before GC invocations=3 (full 1):
 PSYoungGen      total 9216K, used 720K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 70% used [0x00000000fff00000,0x00000000fffb4040,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 8200K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 40% used [0x00000000fe200000,0x00000000fea02010,0x00000000ff600000)
 Metaspace       used 2864K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 两次Minor GC之后空间还是不够，触发Full GC。回收largeOb1内存。
[Full GC (Allocation Failure) [PSYoungGen: 720K->0K(9216K)] [ParOldGen: 8200K->560K(20480K)] 8920K->560K(29696K), [Metaspace: 2864K->2864K(1056768K)], 0.0103751 secs] [Times: user=0.02 sys=0.00, real=0.01 secs] 
Heap after GC invocations=3 (full 1):
 PSYoungGen      total 9216K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 560K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 2% used [0x00000000fe200000,0x00000000fe28c290,0x00000000ff600000)
 Metaspace       used 2864K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb2

Heap
 PSYoungGen      total 9216K, used 246K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 3% used [0x00000000ff600000,0x00000000ff63d890,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 15920K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 77% used [0x00000000fe200000,0x00000000ff18c2a0,0x00000000ff600000)
 Metaspace       used 2870K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 312K, capacity 386K, committed 512K, reserved 1048576K

为什么有两次Minor GC？测试了换其它收集器就不会有。

2、System.gc()，见Minor GC触发条件的第2点。

3、通过Minor GC后进入老年代的平均大小大于老年代的可用内存。

测试代码：

	private static final int _1M = 1024 * 1024;

	/**
	 * 通过Minor GC后进入老年代的平均大小大于老年代的可用内存
	 */
	private static void testFullGCNewAveMore() {

		/* Eden区为8M，from/to space各为1M */
		LargeObject largeOb1 = new LargeObject(_1M * 2, "largeOb1");
		LargeObject largeOb2 = new LargeObject(_1M * 4, "largeOb2");
		LargeObject largeOb3 = new LargeObject(_1M * 2, "largeOb3");
		LargeObject largeOb4 = new LargeObject(_1M * 4, "largeOb4");
		LargeObject largeOb5 = new LargeObject(_1M * 2, "largeOb5");
		LargeObject largeOb6 = new LargeObject(_1M * 4, "largeOb6");
		LargeObject largeOb7 = new LargeObject(_1M * 2, "largeOb7");
	}

	public static void main(String[] agrs) {
		testFullGCNewAveMore();
	}

	static class LargeObject {
		private byte[] data;
		private String name;

		public LargeObject(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObject " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}
	}

测试环境：JDK1.8.0_144，Java HotSpot(TM) 64-Bit Server VM，默认垃圾收集器为PS Scavenge+PS MarkSweep。

-Xms30m -Xmx30m -Xmn10m -XX:+PrintGCDetails -XX:+PrintHeapAtGC

堆总大小为30m，新生代为10m，打印GC信息，在GC前后打印内存信息。

打印结果及分析：

注意Full GC原因，Full GC (Ergonomics) ，意思不太懂。

Over Constructing LargeObject largeOb1

Over Constructing LargeObject largeOb2

{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 7292K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 89% used [0x00000000ff600000,0x00000000ffd1f3f8,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 0K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 0% used [0x00000000fe200000,0x00000000fe200000,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 第一次Minor GC，6152K进入老年代。
[GC (Allocation Failure) [PSYoungGen: 7292K->648K(9216K)] 7292K->6800K(29696K), 0.0028600 secs] [Times: user=0.05 sys=0.02, real=0.00 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 648K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 63% used [0x00000000ffe00000,0x00000000ffea2020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 6152K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 30% used [0x00000000fe200000,0x00000000fe802020,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb3

Over Constructing LargeObject largeOb4

{Heap before GC invocations=2 (full 0):
 PSYoungGen      total 9216K, used 6952K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 76% used [0x00000000ff600000,0x00000000ffc280c0,0x00000000ffe00000)
  from space 1024K, 63% used [0x00000000ffe00000,0x00000000ffea2020,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 6152K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 30% used [0x00000000fe200000,0x00000000fe802020,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 第二次Minor GC，6144K进入老年代。
[GC (Allocation Failure) [PSYoungGen: 6952K->632K(9216K)] 13104K->12928K(29696K), 0.0056290 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
Heap after GC invocations=2 (full 0):
 PSYoungGen      total 9216K, used 632K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 61% used [0x00000000fff00000,0x00000000fff9e030,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 12296K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 60% used [0x00000000fe200000,0x00000000fee02040,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb5

Over Constructing LargeObject largeOb6

{Heap before GC invocations=3 (full 0):
 PSYoungGen      total 9216K, used 6936K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 76% used [0x00000000ff600000,0x00000000ffc28038,0x00000000ffe00000)
  from space 1024K, 61% used [0x00000000fff00000,0x00000000fff9e030,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 12296K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 60% used [0x00000000fe200000,0x00000000fee02040,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 第三次Minor GC，6144K进入老年代。
[GC (Allocation Failure) [PSYoungGen: 6936K->632K(9216K)] 19232K->19072K(29696K), 0.0030912 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=3 (full 0):
 PSYoungGen      total 9216K, used 632K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 61% used [0x00000000ffe00000,0x00000000ffe9e030,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 18440K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 90% used [0x00000000fe200000,0x00000000ff402060,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=4 (full 1):
 PSYoungGen      total 9216K, used 632K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 61% used [0x00000000ffe00000,0x00000000ffe9e030,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 18440K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 90% used [0x00000000fe200000,0x00000000ff402060,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 前三次平均每次进入老年代的大小是6144K，大于老年代剩余大小2040K，触发一次Full GC。
[Full GC (Ergonomics) [PSYoungGen: 632K->0K(9216K)] [ParOldGen: 18440K->18991K(20480K)] 19072K->18991K(29696K), [Metaspace: 2868K->2868K(1056768K)], 0.0577421 secs] [Times: user=0.05 sys=0.00, real=0.06 secs] 
Heap after GC invocations=4 (full 1):
 PSYoungGen      total 9216K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 0% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 18991K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 92% used [0x00000000fe200000,0x00000000ff48bf10,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb7

Heap
 PSYoungGen      total 9216K, used 2372K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 28% used [0x00000000ff600000,0x00000000ff851090,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
  to   space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
 ParOldGen       total 20480K, used 18991K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 92% used [0x00000000fe200000,0x00000000ff48bf10,0x00000000ff600000)
 Metaspace       used 2874K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 312K, capacity 386K, committed 512K, reserved 1048576K

4、Minor GC时，eden space和from space区大小大于to space且大于老年代内存，触发Full GC。

测试代码：

	private static final int _1M = 1024 * 1024;

	/**
	 * Minor GC时，eden space和from space区大小大于to space且大于老年代内存，触发Full GC。
	 */
	private static void testFullGCOldGenLessEden() {

		/* Eden区为8M，from/to space各为1M */
		
		// 8M进入老年代，老年代使用8M
		LargeObject largeOb1 = new LargeObject(_1M * 8, "largeOb1");
	
		// 8M进入老年代，老年代使用16M		
		LargeObject largeOb2 = new LargeObject(_1M * 8, "largeOb2");

		// 4M进入新生代，新生代使用4M		
		LargeObject largeOb3 = new LargeObject(_1M * 4, "largeOb3");
		
		// 4M进入新生代，新生代使用6M
		LargeObject largeOb4 = new LargeObject(_1M * 2, "largeOb4");
		
		// 新生代空间不足，触发MinorGC，新生代的6M老年代装不下，触发Full GC
		LargeObject largeOb5 = new LargeObject(_1M * 1, "largeOb5");
	}

	public static void main(String[] agrs) {
		testFullGCOldGenLessEden();
	}

	static class LargeObject {
		private byte[] data;
		private String name;

		public LargeObject(int size, String name) {
			data = new byte[size];
			this.name = name;
			System.out.println("Over Constructing LargeObject " + name + System.lineSeparator());
		}

		public String getName() {
			return name;
		}
	}

测试环境：JDK1.8.0_144，Java HotSpot(TM) 64-Bit Server VM，默认垃圾收集器为PS Scavenge+PS MarkSweep。

-Xms30m -Xmx30m -Xmn10m -XX:+PrintGCDetails -XX:+PrintHeapAtGC

堆总大小为30m，新生代为10m，打印GC信息，在GC前后打印内存信息。

打印结果及分析：

注意Full GC原因，Full GC (Ergonomics) ，意思不太懂。

Over Constructing LargeObject largeOb1

Over Constructing LargeObject largeOb2

Over Constructing LargeObject largeOb3

Over Constructing LargeObject largeOb4

{Heap before GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 7292K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 89% used [0x00000000ff600000,0x00000000ffd1f0a0,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 16384K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 80% used [0x00000000fe200000,0x00000000ff200020,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 申请largeOb5时，eden space剩余900K，不足1024K，所以触发Minor GC。
[GC (Allocation Failure) --[PSYoungGen: 7292K->7292K(9216K)] 23676K->23684K(29696K), 0.0012875 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
Heap after GC invocations=1 (full 0):
 PSYoungGen      total 9216K, used 7292K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 89% used [0x00000000ff600000,0x00000000ffd1f0a0,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 63% used [0x00000000ffe00000,0x00000000ffea2020,0x00000000fff00000)
 ParOldGen       total 20480K, used 16392K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 80% used [0x00000000fe200000,0x00000000ff202020,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=2 (full 1):
 PSYoungGen      total 9216K, used 7292K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 89% used [0x00000000ff600000,0x00000000ffd1f0a0,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 63% used [0x00000000ffe00000,0x00000000ffea2020,0x00000000fff00000)
 ParOldGen       total 20480K, used 16392K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 80% used [0x00000000fe200000,0x00000000ff202020,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K

// 上一次Minor GC时，由于eden space+from space大小大于老年代剩余大小，不能分配到老年代，触发Full GC。
[Full GC (Ergonomics) [PSYoungGen: 7292K->6703K(9216K)] [ParOldGen: 16392K->16385K(20480K)] 23684K->23088K(29696K), [Metaspace: 2868K->2868K(1056768K)], 0.0078789 secs] [Times: user=0.05 sys=0.00, real=0.01 secs] 
Heap after GC invocations=2 (full 1):
 PSYoungGen      total 9216K, used 6703K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 81% used [0x00000000ff600000,0x00000000ffc8bea0,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 16385K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 80% used [0x00000000fe200000,0x00000000ff200540,0x00000000ff600000)
 Metaspace       used 2868K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 311K, capacity 386K, committed 512K, reserved 1048576K
}
Over Constructing LargeObject largeOb5

Heap
 PSYoungGen      total 9216K, used 7973K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 97% used [0x00000000ff600000,0x00000000ffdc9740,0x00000000ffe00000)
  from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
  to   space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
 ParOldGen       total 20480K, used 16385K [0x00000000fe200000, 0x00000000ff600000, 0x00000000ff600000)
  object space 20480K, 80% used [0x00000000fe200000,0x00000000ff200540,0x00000000ff600000)
 Metaspace       used 2874K, capacity 4490K, committed 4864K, reserved 1056768K
  class space    used 312K, capacity 386K, committed 512K, reserved 1048576K

5、Metaspace空间不足。不知如何演示。

相关标签： finalize JVM Minor GC GC Full GC

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