Java OutOfMemoryError even with Correct Structuring?

Question

I am working with 2D int arrays in Java, that are square and can have ~30000 elements in a row and column, which means there are 30000^2*4 bytes in the array, which is less than 5GB (and I have much more than 5GB of memory available).

My basic structure of the program is this:

public class A {
    public static void main(String[] args) {
        Graph g = ...; // read from file
        System.out.println(B.computeSomethingBig(g));
    }
}

public class B {
     public static computeSomethingBig(Graph g) {
         int numVertices = g.numVertices();
         System.out.println(numVertices); // ~30000 maximum
         int[][] array = new int[numVertices][numVertices];
         // ... other computations
     }
}

Now, in Eclipse, I am running main in class A with these arguments:

-Xms5g -Xmx10g

I have numVertices print out a value around 30000, and setting the minimum heap size (-Xms) seems more than necessary. However, I get:

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

You are declaring an array with 30 000 integers with 30 000 integers inside each of them — nbro
– nbro, Commented Aug 22, 2014 at 3:24
@Thilo new T[int_value][int_value] initializes both dimensions (new T[int_value][/*empty*/] would do only first) and @Ryan int is indeed 4 bytes. Creating (only) int[30000][30000] works for me, in 7u55 on x64 with -Xmx4G, and Runtime .totalMemory() - .freeMemory() confirms it uses slightly over 3.6e9 bytes. — dave_thompson_085
– dave_thompson_085, Commented Aug 22, 2014 at 6:14

triggerNZ · Accepted Answer · 2014-08-22 03:46:20Z

3

Running the following:

public class A {
        public static void main(String[] args) {
                int numVertices = 30000;
                int[][] array = new int[numVertices][numVertices];
        }
}

without any parameters, ie java A leads to OOM error. Running java -Xms5g -Xmx10g A works. I suspect numVertices is bigger than what you expected. Why don't you output it before allocating to make sure.

Also consider that your Graph, or other objects in the application could be using heap space too.

edited Aug 22, 2014 at 3:46

answered Aug 22, 2014 at 3:27

triggerNZ

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9 Comments

Ryan Dougherty Over a year ago

So basically, the heap is only specific to the main method it was called from? and not any method that it calls (maybe in some other class)?

jdphenix Over a year ago

@Ryan No, the Java heap is the space reserved for objects in your application. docs.oracle.com/cd/E13150_01/jrockit_jvm/jrockit/geninfo/…

Ryan Dougherty Over a year ago

So then why would the solution you have work and not mine? I am creating a 2D array in the same manner, just in a different method.

Thilo Over a year ago

Also consider a one-dimensional array instead. There is some overhead involved in a two-dimensional array (as opposed to manually mapping coordinated into a one-dimensional array). At these sizes, this may matter.

Ryan Dougherty Over a year ago

@jdphenix Interesting - running that gives both InitialHeapSize and MaxHeapSize to be 2^32 (4294967296).

|

Flexo - Save the data dump · Accepted Answer · 2015-05-06 21:35:00Z

1

What you seem to face is the case where no individual area inside heap (such as eden, survivor or old) is big enough to accommodate the data structure you are trying to allocate. For example - your 5G heap can by default be divided as following (note that this is platform specific):

Eden 1.5G
Two Survivor spaces 0.25G each
Old 3G

More fine-grained configuration by tweaking the ratios of the different areas will sort the things out for you.

edited May 6, 2015 at 21:35

Flexo - Save the data dump♦

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answered Aug 22, 2014 at 12:44

Ivo

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