Java requires larger heap space than what is reasonable

Question

I have a program where I will be using a very large short[] array:

import java.lang.Math;

public class HPTest {

    public static void main(String[] args) {
        int n = 30;
        short[] a = new short[(int)Math.pow(2,n)];
    }
}

As far as I know, a short[] array should use 2 bytes per element, and so an array with 2^30 elements should need about 2 GiB of RAM.

In order to run the program, I therefore tried

java -Xms2000m HPTest

but still got a heap space error. Even at 3000m I got the same error, but at 4000m it worked.

Any ideas as to why I had to go so far above the estimated limit of 2000m?

EDIT: As has been pointed out by many users, I made a very embarrassing error in declaring that a short needs 1 byte rather than 2 bytes. The question then should be why it doesn't suffice with 2000m.

Answer 1

Something this large, will be much happier outside the heap. You would be better off looking in to NIO and using direct byte buffers to back your large Short array. This memory can be kept out of the heap, and away from the mitts of the garbage collector (who may at times feel inclined to want to copy your buffer from one area to the other).

See java.nio.ShortBuffer and start digging from there.