java.lang.OutOfMemoryError: Java heap space error not solved by -Xmx and -Xms

Question

I'm beginner in java and I tried to make a code which sums two matrixes, but I have the error :

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at Main.main(Main.java:11)

I tried all this commands :

$ java Main
$ java Main -Xmx2048m
$ java Main -Xms1024m
$ java Main -Xms1024m -Xmx2048m
$ java -Xmx2048m Main
$ java -Xms1024m Main
$ java -Xms1024m -Xmx2048m Main

But all give the same error message...

This is the code :

public class Main
{
    public static void main(String[] args)
    {
        final int n = 10000;

        double A[][] = new double[n][n];
        double B[][] = new double[n][n];
        double S[][] = new double[n][n];

        int i,j;

        for (i = 0; i<n; i++)
        {
            for (j = 0; j<n; j++)
            {
                A[i][j] = 1.0d / ((double) i*j);
                B[i][j] = 1.0d / ((double) i*j);
            }
        }

        for (i = 0; i<n; i++)
        {
            for (j = 0; j<n; j++)
            {
                S[i][j] = A[i][j] + B[i][j];
            }
        }
    }
}

So as you can see, this code need ~ 3x8x10000 B = 240 000 B < 2048 MB. My java's version :

$ java -version
openjdk version "11.0.4" 2019-07-16
OpenJDK Runtime Environment (build 11.0.4+11-post-Ubuntu-1ubuntu218.04.3)
OpenJDK 64-Bit Server VM (build 11.0.4+11-post-Ubuntu-1ubuntu218.04.3,     mixed mode, sharing)

You are trying to initialize very huge 2-D array. You can try with n =100. — dSanders
– dSanders, Commented Sep 21, 2019 at 15:43
I was able to run your program in my eclipse. But my machine has 16 GB RAM. With out setting -Xmx2048m and -Xms it is working fine — Sujay Mohan
– Sujay Mohan, Commented Sep 21, 2019 at 16:22

Simon Kocurek · Accepted Answer · 2019-09-21 15:52:04Z

1

The flags work, but your math does not check out.

You have 3 arrays, each with 10 000 * 10 000 * 8B values.
For that you need 3 * 10 000 * 10 000 * 8 Bytes... which is 2289 MB.
That is already more than 2048.

To add to that, each of those 3 arrays contains 10 000 references... but the memory overhead of that is minuscule (less than 1MB) compared to the values.

answered Sep 21, 2019 at 15:52

Simon Kocurek

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1 Comment

anthomedal Over a year ago

Oh yes... I'm stupid --' (and I'im not computer scientist : I'im mathematician !) Thank You !

Sujay Mohan · Accepted Answer · 2019-09-21 16:24:22Z

0

I am able to run your code without any Xms or Xmx setting. My Laptop is having 16 GB Ram.

answered Sep 21, 2019 at 16:24

Sujay Mohan

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