在 64 位操作系统中，Jvm 从何处获取堆内存？

Question

Am getting 104 Gp heap memory using command -Xms500m -Xmx139000m

Am usig core i5 processor 64 bit Windows 7 Os. 500 Gp harddisk. 4GB RAM Only

1. I Just want to know from where Jvm takes memory of 104 GB(heap) ?

2. In the output no memmory usage is displayed?

    `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(); } } `

   And the output is

    `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

The options "-Xms500m -Xmx139000m" mean "allocate an initial heap size of 500Mb, and let it grow to a maximum of 139GB ... if it needs to".

The output you are seeing from your program is entirely consistent with that. At the point the program ran, the heap had not reached 139Gb. And it might never reach that level. And it may not even be able to reach that level ... depending on the resources that the operating system is able to give the JVM if / when it asks for them.

If you really want to force the JVM to use a 139Gb heap, you should try setting 139Gb as the initial heap size too; eg "-Xms139000m -Xmx139000m". But that's probably not a good idea, especially if you don't have that much physical RAM.

Answer 2

Virtual memory means RAM is allocated on usage, not when the address space is reserved. If you actually use the 104GB, the OS will use swap (a file or partition on disk) to maintain the illusion of more RAM than you physically have.