A simple "Hello World" needs 10G virtual memory on a 64-bit machine vs 1G at 32-bit?

Question

Running a simple Java program on our production machine, I noticed that this program eats up more 10G virt. I know that virtual memory is not that relevant, but at least I would like to understand why this is needed.

public class Main {
  public static void main(String[] args) {
        System.out.println("Hello World!");
        try {
                Thread.sleep(10000);
        } catch(InterruptedException e) {
                /* ignored */
        }
  }
}

Heres what top is saying when i run that little program:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
18764 myuser    20   0 10.2g  20m 8128 S  1.7  0.1   0:00.05 java

Does anyone know why this is happening?

uname -a says:

Linux m4fxhpsrm1dg 2.6.32-358.18.1.el6.x86_64 #1 SMP Fri Aug 2 17:04:38 EDT 2013 x86_64 x86_64 x86_64 GNU/Linux

On an older 32bit-linux machine the same program consumes only about 1G virt. The old machine has 4GB RAM, the new one 32GB.

Answer 1

The default sizes for initial heap and maximum heap are defined as a percentage of the machine's physical memory, of which a production server nowadays tends to have a whole lot.

You can choose both via the -Xms and -Xmx command line options.