jvm GC：垃圾回收的测试与分析
实验环境：
（1）Java版本以及模式：
java version "1.8.0_171"
Java(TM) SE Runtime Environment (build 1.8.0_171-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.171-b11, mixed mode)
（2）垃圾回收器为Parrallel Scanvenge收集器 和 Parrallel Old收集器
（3）CPU：Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz
1、查看基础内存消耗
public class GCTest { public static void main(String[] args){} }
//使用javac编译并执行java -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 GCTest
// 日志如下：
Heap
PSYoungGen total 9216K, used 1312K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 16% used [0x00000000ff600000,0x00000000ff748228,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 10240K, used 0K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
object space 10240K, 0% used [0x00000000fec00000,0x00000000fec00000,0x00000000ff600000)
Metaspace used 2538K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 278K, capacity 386K, committed 512K, reserved 1048576K
分析日志可知：eden区基础消耗1312K的内存，其中新生代大小为9216K = 8192 + 1024 K，因为 to 区和 from 区只有其中一个能使用。我并不知道基础消耗到哪里去了？有知道的大神告诉我下。
注意：使用cmd命令行执行，基础内存消耗较低，原先使用idea执行基础内存消耗达到2M。
2、新创建的对象内存大于新生代剩余内存将直接分配到老年代
import sun.misc.Unsafe; import java.lang.reflect.Field; public class GCTest { private static Unsafe unsafe; static { try { Field field = Unsafe.class.getDeclaredField("theUnsafe"); field.setAccessible(true); unsafe = (Unsafe) field.get(null); } catch (Exception e) { e.printStackTrace(); } } public static long addressOf(Object o) throws Exception { Object[] array = new Object[] { o }; long baseOffset = unsafe.arrayBaseOffset(Object[].class); int addressSize = unsafe.addressSize(); long objectAddress; switch (addressSize) { case 4: objectAddress = unsafe.getInt(array, baseOffset); break; case 8: objectAddress = unsafe.getLong(array, baseOffset); break; default: throw new Error("unsupported address size: " + addressSize); } return (objectAddress); } private static final int _1MB = 1024 * 1024; public static void main(String[] args) throws Exception { byte[] allocation1, allocation2, allocation3, allocation4; allocation1 = new byte[2 * _1MB]; allocation2 = new byte[2 * _1MB]; allocation3 = new byte[2 * _1MB]; allocation4 = new byte[4 * _1MB];
// 打印对象所在的起始内存地址 System.out.println("allocation1:0x00000000" + Long.toHexString(addressOf(allocation1))); System.out.println("allocation2:0x00000000" + Long.toHexString(addressOf(allocation2))); System.out.println("allocation3:0x00000000" + Long.toHexString(addressOf(allocation3))); System.out.println("allocation4:0x00000000" + Long.toHexString(addressOf(allocation4))); } }
//使用javac编译并执行java -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 GCTest
// 日志如下：
allocation1:0x00000000ff71f158 // 处于新生代
allocation2:0x00000000ff91f168 // 处于新生代
allocation3:0x00000000ffb1f178 // 处于新生代
allocation4:0x00000000fec00000 // 处于老年代
Heap
PSYoungGen total 9216K, used 7456K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 91% used [0x00000000ff600000,0x00000000ffd48258,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
to space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
ParOldGen total 10240K, used 4096K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
object space 10240K, 40% used [0x00000000fec00000,0x00000000ff000010,0x00000000ff600000)
Metaspace used 2568K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 284K, capacity 386K, committed 512K, reserved 1048576K
分析日志可知：Minor GC没有触发，由于新生代剩余大小也放不下allocation4，于是allocation4直接分配到老年代中。
3、Minor GC的触发与过程
import sun.misc.Unsafe; import java.lang.reflect.Field; public class GCTest { private static Unsafe unsafe; static { try { Field field = Unsafe.class.getDeclaredField("theUnsafe"); field.setAccessible(true); unsafe = (Unsafe) field.get(null); } catch (Exception e) { e.printStackTrace(); } } public static long addressOf(Object o) throws Exception { Object[] array = new Object[] { o }; long baseOffset = unsafe.arrayBaseOffset(Object[].class); int addressSize = unsafe.addressSize(); long objectAddress; switch (addressSize) { case 4: objectAddress = unsafe.getInt(array, baseOffset); break; case 8: objectAddress = unsafe.getLong(array, baseOffset); break; default: throw new Error("unsupported address size: " + addressSize); } return (objectAddress); } private static final int _1MB = 1024 * 1024; public static void main(String[] args) throws Exception { byte[] allocation1, allocation2, allocation3, allocation4; allocation1 = new byte[2 * _1MB]; allocation2 = new byte[2 * _1MB]; allocation3 = new byte[3 * _1MB]; // 触发Minor GC allocation4 = new byte[2 * _1MB]; System.out.println("allocation1:0x00000000" + Long.toHexString(addressOf(allocation1))); System.out.println("allocation2:0x00000000" + Long.toHexString(addressOf(allocation2))); System.out.println("allocation3:0x00000000" + Long.toHexString(addressOf(allocation3))); System.out.println("allocation4:0x00000000" + Long.toHexString(addressOf(allocation4))); } }// 使用javac编译并执行java -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 GCTest
// 日志如下：
[GC (Allocation Failure) [PSYoungGen: 5244K->768K(9216K)] 5244K->4872K(19456K), 0.0031067 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
allocation1:0x00000000fec00000 // 处于老年代
allocation2:0x00000000fee02010 // 处于老年代
allocation3:0x00000000ff600000 // 处于新生代
allocation4:0x00000000ff900010 // 处于新生代
Heap
PSYoungGen total 9216K, used 6129K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 65% used [0x00000000ff600000,0x00000000ffb3c410,0x00000000ffe00000)
from space 1024K, 75% used [0x00000000ffe00000,0x00000000ffec0030,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 10240K, used 4104K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
object space 10240K, 40% used [0x00000000fec00000,0x00000000ff002020,0x00000000ff600000)
Metaspace used 2569K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 284K, capacity 386K, committed 512K, reserved 1048576K
分析日志：
（1）进行了一次Minor GC，其中新生代占用大小从5244K 减少为 768K，总占用从5244K 减少为 4872K，清理掉的内存大小 5244 - 4872 = 372K，则老年代大小为 5244 - 768 - 372 = 4104K，该值与最终的老年代大小占用相同。新生代的768K为from space持有，因为1024 * 75% = 768K.
（2） Minor GC的触发条件：当Eden区域放不下新创建的对象，但是新生代的剩余空间大于新创建的对象大小时。
放入allocation1和allocation2时： 1314K（基础消耗）+ 2 * 1024K（allocation1）+ 2*1024K（allocation2）= 5410K 小于Eden区
放入allocation3时： 5410K + 3072K = 8482K 大于Eden区域（8192K） 小于新生代大小（9216K） 此时触发Minor GC
Minor GC的过程：
（1）将存活对象从Eden区将对象转入from区域（如：上述from space的内存占用为768K）。
（2）转移期间，对象太大，将直接进入老年代（如：上述老年代的4104K的内存大小）。
（3）转移后，非存活对象将被清理掉（如：上述372K的内存大小）。
4、Full GC触发与过程
import sun.misc.Unsafe; import java.lang.reflect.Field; public class GCTest { private static Unsafe unsafe; static { try { Field field = Unsafe.class.getDeclaredField("theUnsafe"); field.setAccessible(true); unsafe = (Unsafe) field.get(null); } catch (Exception e) { e.printStackTrace(); } } public static long addressOf(Object o) throws Exception { Object[] array = new Object[] { o }; long baseOffset = unsafe.arrayBaseOffset(Object[].class); int addressSize = unsafe.addressSize(); long objectAddress; switch (addressSize) { case 4: objectAddress = unsafe.getInt(array, baseOffset); break; case 8: objectAddress = unsafe.getLong(array, baseOffset); break; default: throw new Error("unsupported address size: " + addressSize); } return (objectAddress); } private static final int _1MB = 1024 * 1024; public static void main(String[] args) throws Exception { byte[] allocation1, allocation2, allocation3, allocation4; allocation1 = new byte[2 * _1MB]; allocation2 = new byte[3 * _1MB]; // 转入老年代时，由于allocation2 大于 allocation1，所以触发了Full GC allocation3 = new byte[2 * _1MB]; // 触发Minor GC System.out.println("allocation1:0x00000000" + Long.toHexString(addressOf(allocation1))); System.out.println("allocation2:0x00000000" + Long.toHexString(addressOf(allocation2))); System.out.println("allocation3:0x00000000" + Long.toHexString(addressOf(allocation3))); } }// 使用javac编译并执行java -verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 GCTest
日志如下：
[GC (Allocation Failure) [PSYoungGen: 6268K->768K(9216K)] 6268K->5896K(19456K), 0.0036739 secs] [Times: user=0.01 sys=0.05, real=0.02 secs]
[Full GC (Ergonomics) [PSYoungGen: 768K->0K(9216K)] [ParOldGen: 5128K->5750K(10240K)] 5896K->5750K(19456K), [Metaspace: 2559K->2559K(1056768K)],
0.0068489 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
allocation1:0x00000000fec00000 // 处于老年代
allocation2:0x00000000fee004e8 // 处于老年代
allocation3:0x00000000ff600000 // 处于新生代
Heap
PSYoungGen total 9216K, used 2289K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 8192K, 27% used [0x00000000ff600000,0x00000000ff83c770,0x00000000ffe00000)
from space 1024K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x00000000fff00000)
to space 1024K, 0% used [0x00000000fff00000,0x00000000fff00000,0x0000000100000000)
ParOldGen total 10240K, used 5750K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
object space 10240K, 56% used [0x00000000fec00000,0x00000000ff19d8e8,0x00000000ff600000)
Metaspace used 2569K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 284K, capacity 386K, committed 512K, reserved 1048576K
分析日志：
（1）发生一次Minor GC与一次Full GC，第一次Minor GC回收了 6268K - 5896K = 372K 内存，新生代剩余768K内存，这一步和上一个例子相同。
（2）由于allocation1和allocation2的大小大于from space区，所以需要直接转入老年代。但由于allocation2大于allocation1，触发Full GC来防止老年代的空间不足以存放新的对象。
这次GC回收了5896K - 5750K = 146K的内存空间，新生代内存占用变为0K，老年代内存占用添加768K - 146K = 622K。老年代内存占用= 622K+allocation1(2*1024)+allocation2(3*1024) = 5742K (约等于5750K)
多出来的8K，我也不知道在哪里了，有大神知道请告诉我。
（3）allocation3直接放入了Eden区域，新生代最终内存占用为 2048K(约等于2289K)。
Full GC的过程：
（1）一般会触发一次Minor GC，即from space区对象的存活时间较长能够升入老年代。
（2）老年代垃圾回收，将非存活对象回收。
5、总结
（1）Minor GC触发条件：新创建的对象大于Eden区剩余空间，小于新生代剩余空间时触发；Full GC时伴随触发一次。
（2）Full GC触发条件：由新生代转入老年代时，对象内存占用大于平均转入老年代的对象内存占用。
