java - 64位操作系统中Jvm从哪里获取堆内存？

Question

使用命令 -Xms500m -Xmx139000m 获取 104 Gp 堆内存

我使用的是酷睿 i5 处理器 64 位 Windows 7 操作系统。 500 Gp 硬盘。仅 4GB 内存

1. 我只是想知道Jvm从哪里获取104GB(堆)的内存？

2. 输出中没有显示内存使用情况？

            `public class CPUusage {
        public static void main(String[] args)throws Exception
        {
   int mb = 1024*1024;
   int GB = 1024*1024*1024;
   /* Total number of processors or cores available to the JVM */    
   System.out.println("Available processors (cores): " +          Runtime.getRuntime().availableProcessors());
   /* Total amount of free memory available to the JVM */ 
   System.out.println("Free memory (MB): " +          Runtime.getRuntime().freeMemory()/mb);
   /* This will return Long.MAX_VALUE if there is no preset limit */  
   long maxMemory = Runtime.getRuntime().maxMemory()/GB;
   /* Maximum amount of memory the JVM will attempt to use */    
   System.out.println("Maximum memory (GB): " +          maxMemory);
   /* Total memory currently in use by the JVM */  
   System.out.println("Total memory (MB): " +          Runtime.getRuntime().totalMemory()/mb);
   /* Get a list of all filesystem roots on this system */
   File log=new File("D:\\log.txt");
   log.createNewFile();
   FileWriter fstream = new FileWriter(log);
   BufferedWriter out = new BufferedWriter(fstream);
     out.write("--------------------------------------------"+"\n\n");
   
   out.write("Available processors (cores): " +          Runtime.getRuntime().availableProcessors());
   out.newLine();
   out.write("Free memory (MB): " +          Runtime.getRuntime().freeMemory()/mb);
   out.newLine();
   out.write("Maximum memory (MB): " +          (maxMemory == Long.MAX_VALUE ? "no limit" : maxMemory));
   out.newLine();
   out.write("Total memory (MB): " +          Runtime.getRuntime().totalMemory()/mb); 
   out.newLine();
   
   File[] roots = log.listRoots();
   /* For each filesystem root, print some info */ 
   for (File root : roots) {
       System.out.println("-------------------------------------------");
       System.out.println("File system root: " + root.getAbsolutePath());
       System.out.println("Total space (GB): " + root.getTotalSpace()/GB);
       System.out.println("Free space (GB): " + root.getFreeSpace()/GB);
       System.out.println("Usable space (GB): " + root.getUsableSpace()/GB);
      out.write("-------------------------------------------");
       out.newLine();
       out.write("File system root: " + root.getAbsolutePath());
       out.newLine();
       out.write("Total space (GB): " + root.getTotalSpace()/GB);
       out.newLine();
       out.write("Free space (GB): " + root.getFreeSpace()/GB);
       out.newLine();
       out.write("Usable space (GB): " + root.getUsableSpace()/GB);
       out.newLine();
   
   }
     out.write("-------------------------------------------");
     out.newLine();
     out.close();
       }
       } 
   
              `
   

   输出是

            `Available processors (cores): 4
             Free memory (MB): 476
             Maximum memory (GB): 104
             Total memory (MB): 479
             -------------------------------------------
             File system root: C:\
             Total space (GB): 97
             Free space (GB): 70
             Usable space (GB): 70
             -------------------------------------------
             File system root: D:\
             Total space (GB): 368
             Free space (GB): 366
             Usable space (GB): 366
   
             `
   

Answer 1

选项“-Xms500m -Xmx139000m”的意思是“分配500Mb的初始堆大小，并让它增长到最大139GB......如果需要的话”。

您从程序中看到的输出与此完全一致。在程序运行时，堆还没有达到 139Gb。它可能永远达到那个水平。它甚至可能无法达到该级别……具体取决于操作系统在 JVM 需要时能够提供的资源。

如果您确实想强制 JVM 使用 139Gb 堆，您也应该尝试将初始堆大小设置为 139Gb；例如“-Xms139000m -Xmx139000m”。但这可能不是一个好主意，特别是如果您没有那么多物理 RAM。