Question

使用命令-Xms500m -Xmx139000m

获得104 Gp堆内存

我们的核心i5处理器64位Windows 7 Os。 500 Gp硬盘。仅限4GB RAM

我只是想知道Jvm从哪里获取104 GB（堆）的内存？

在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

         `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”表示“分配初始堆大小为500Mb，并让它增长到最大139GB ......如果需要”。

您从程序中看到的输出与此完全一致。在程序运行时，堆没有达到139Gb。它可能从未达到该水平。它甚至可能能够达到该级别......取决于操作系统能够在何时/何时请求它们的情况下为JVM提供的资源。

如果你真的想强制JVM使用139Gb堆，你应该尝试将139Gb设置为初始堆大小;例如“-Xms139000m -Xmx139000m”。但这可能不是一个好主意，特别是如果你没有那么多的物理RAM。

Answer 2

虚拟内存意味着RAM在使用时分配，而不是在保留地址空间时分配。如果您实际使用的是104GB，则操作系统将使用swap（磁盘上的文件或分区）来保持比您实际拥有的RAM更多的错觉。

从Jvm在64位操作系统中为Heap占用内存的地方？

2 个答案: