jvm参数PretenureSizeThreshold让对象直接进入老年代！

VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:PretenureSizeThreshold=3145728

/**
 * @author: csh
 * @Date: 2020/12/9 16:27
 * @Description:避免在Eden区及两个Survivor区之间来回复制，产生大量的内存复制操作。
 * 注：-XX：PretenureSizeThreshold参数只对Serial和ParNew两款新生代收集器有效，HotSpot的其他新生代收集器，如Parallel Scavenge并不支持这个参数。
 */
public class OldGC {
    private static final int _1MB = 1024 * 1024;
    /**
     * VM参数：-verbose:gc -Xms20M -Xmx20M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:PretenureSizeThreshold=3145728
     */
    public static void testPretenureSizeThreshold() {
        byte[] allocation;
        allocation = new byte[4 * _1MB]; //直接分配在老年代中
    }

    public static void main(String[] args) {
        testPretenureSizeThreshold();
    }
}

Heap
 PSYoungGen total 9216K, used 6302K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 8192K, 76% used [0x00000000ff600000,0x00000000ffc27848,0x00000000ffe00000)
  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 3399K, capacity 4496K, committed 4864K, reserved 1056768K
  class space used 370K, capacity 388K, committed 512K, reserved 1048576K