package cn.itcast.jvm.t2;
import java.util.ArrayList;
/**
 *  演示內(nèi)存的分配策略
 */
public class Demo2_1 {
    private static final int _512KB = 512 * 1024;
    private static final int _1MB = 1024 * 1024;
    private static final int _6MB = 6 * 1024 * 1024;
    private static final int _7MB = 7 * 1024 * 1024;
    private static final int _8MB = 8 * 1024 * 1024;
//加入Java開發(fā)交流君樣：756584822一起吹水聊天
    // -Xms20M -Xmx20M -Xmn10M : 堆初始與最大大小都是20M，新生代的大小為10M.
    // -XX:+UseSerialGC ： 為了學習方便，采用這個垃圾回收器，默認的垃圾回收器并不是這個。
    // -XX:+PrintGCDetails -verbose:gc ：打印詳細信息
    // -XX:-ScavengeBeforeFullGC ：在Full GC 前進行 Minor GC.
    public static void main(String[] args) throws InterruptedException {
        new Thread(() -> {
            ArrayList<byte[]> list = new ArrayList<>();
            list.add(new byte[_8MB]);
            list.add(new byte[_8MB]);
        }).start();
        System.out.println("sleep....");
        Thread.sleep(1000L);
    }
}

Heap
 def new generation   total 9216K, used 2341K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
// 新生代的空間總的大小為9216K,這里沒有把To空間給計算進去，系統(tǒng)任務To的空間是分配是不可用的，所以不是10M,已經(jīng)使用了2341K，[]內(nèi)部則是內(nèi)存地址范圍。                              
  eden space 8192K,  28% used [0x00000000fec00000, 0x00000000fee49420, 0x00000000ff400000)
//                                                          
  from space 1024K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
  to   space 1024K,   0% used [0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)
 tenured generation   total 10240K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
// 老年代大小為10M，可以看到?jīng)]有任何空間使用                                             
                                          //加入Java開發(fā)交流君樣：756584822一起吹水聊天   
   the space 10240K,   0% used [0x00000000ff600000, 0x00000000ff600000, 0x00000000ff600200, 0x0000000100000000)
 Metaspace       used 3394K, capacity 4496K, committed 4864K, reserved 1056768K
  class space    used 378K, capacity 388K, committed 512K, reserved 1048576K

    public static void main(String[] args) throws InterruptedException {
        ArrayList<byte[]> list = new ArrayList<>();
        list.add(new byte[_7MB]);      // 系統(tǒng)類占用2341K，加上new的7MB觸發(fā)垃圾回收
    }

[GC (Allocation Failure) [DefNew: 2342K->696K(9216K), 0.0029193 secs] 2342K->696K(19456K), 0.0029867 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
// GC:minor GC(新生代垃圾回收)    FUll GC  (老年代垃圾回收)
// [Times: user=0.00 sys=0.00, real=0.00 secs] 垃圾回收執(zhí)行時間
Heap
 def new generation   total 9216K, used 8110K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 8192K,  90% used [0x00000000fec00000, 0x00000000ff33d8c0, 0x00000000ff400000)
  from space 1024K,  67% used [0x00000000ff500000, 0x00000000ff5ae100, 0x00000000ff600000)
  to   space 1024K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
 tenured generation   total 10240K, used 0K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,   0% used [0x00000000ff600000, 0x00000000ff600000, 0x00000000ff600200, 0x0000000100000000)
 Metaspace       used 3539K, capacity 4536K, committed 4864K, reserved 1056768K
 //加入Java開發(fā)交流君樣：756584822一起吹水聊天
  class space    used 395K, capacity 428K, committed 512K, reserved 1048576K

    public static void main(String[] args) throws InterruptedException {
        ArrayList<byte[]> list = new ArrayList<>();
        list.add(new byte[_7MB]);
        list.add(new byte[_512KB]);
        list.add(new byte[_512KB]);
    }

[GC (Allocation Failure) [DefNew: 2342K->670K(9216K), 0.0022591 secs] 2342K->670K(19456K), 0.0023131 secs] [Times: user=0.02 sys=0.00, real=0.00 secs] 
[GC (Allocation Failure) [DefNew: 8678K->538K(9216K), 0.0061246 secs] 8678K->8354K(19456K), 0.0061637 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
Heap
 def new generation   total 9216K, used 1132K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 8192K,   7% used [0x00000000fec00000, 0x00000000fec94930, 0x00000000ff400000)
  from space 1024K,  52% used [0x00000000ff400000, 0x00000000ff486a00, 0x00000000ff500000)
  to   space 1024K,   0% used [0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)
 tenured generation   total 10240K, used 7815K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  76% used [0x00000000ff600000, 0x00000000ffda1f80, 0x00000000ffda2000, 0x0000000100000000)
 Metaspace       used 3539K, capacity 4536K, committed 4864K, reserved 1056768K
 //加入Java開發(fā)交流君樣：756584822一起吹水聊天
  class space    used 395K, capacity 428K, committed 512K, reserveed 1048576K

    public static void main(String[] args) throws InterruptedException {
        ArrayList<byte[]> list = new ArrayList<>();
        list.add(new byte[_8MB]);
    }

Heap
 def new generation   total 9216K, used 2507K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
  eden space 8192K,  30% used [0x00000000fec00000, 0x00000000fee72ca8, 0x00000000ff400000)
  from space 1024K,   0% used [0x00000000ff400000, 0x00000000ff400000, 0x00000000ff500000)
  to   space 1024K,   0% used [0x00000000ff500000, 0x00000000ff500000, 0x00000000ff600000)
 tenured generation   total 10240K, used 8192K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
   the space 10240K,  80% used [0x00000000ff600000, 0x00000000ffe00010, 0x00000000ffe00200, 0x0000000100000000)
 Metaspace       used 3539K, capacity 4536K, committed 4864K, reserved 1056768K
  class space    used 395K, capacity 428K, committed 512K, reserved 1048576K

-XX:+UseSerialGC    = Serial + SerialOld
// Serial：工作在新生代，采用復制的垃圾回收算法
// SerialOld:工作在老生代，采用標記+整理的垃圾回收算法

-XX:+UseParallelGC , -XX:+UseParallelOldGC 

-XX:+UseAdaptiveSizePolicy 

-XX:GCTimeRatio=ratio          

-XX:MaxGCPauseMillis=ms   

-XX:+UseConcMarkSweepGC // use concurrent mark sweep（會產(chǎn)生垃圾碎片） 工作在老年代的垃圾回收器
-XX:+UseParNewGC        // 工作在新生代的垃圾回收器

-XX:ParallelGCThreads=n        // 并行的垃圾回收線程數(shù)，通常等于CPU的核心數(shù)（垃圾回收并行階段）
-XX:ConcGCThreads=threads      // 并發(fā)的線程數(shù)目，通常設為并行垃圾回收線程數(shù)的1/4（垃圾回收并發(fā)階段）

-XX:CMSInitiatingOccupancyFraction=percent // 執(zhí)行垃圾回收的內(nèi)存占比，預留空間給浮動垃圾
-XX:+CMSScavengeBeforeRemark
// 在重新標記前，對新生代進行垃圾回收，減少并發(fā)清理的垃圾對象，+開啟，-關閉