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javajvmjvm-arguments

about JVM heap argument -Xmx and -Xms

I have following code:

public static void main(String[] args) {
    for(int i=0;i<10;i++) {
        System.out.println(i);
        byte[] b = new byte[1024*1024*5];
    }
}

You see each operation allocates 5M. When I set -Xms8M -Xmx8M, it runs successfully without exception, while when -Xms7M -Xmx7M, it throws OutOfMemoryError exception. Could someone explain why? I'm under Windows 7, 64bit, Eclipse 4.3. Following code is same result:

public static void main(String[] args) {
    byte[] b;
    for(int i=0;i<10;i++) {
        System.out.println(i);
        b = new byte[1024*1024*5];
    }
}
Solution

  • You mistakenly assume that only 5MB of memory are allocated by that program, and you're wrong. Just because you create a 5MB array doesn't mean there isn't additional memory used for classloading and other things.

    So to answer your question, you're allocating too little memory for your process.