Heap max limit

Question

Hello, I have done some tests to check the heap size of an application.
This is my test code:
public class Main {
     public static void main(String[] args) {
          Runtime runtime = Runtime.getRuntime();
          // Max limit for heap allocation.
          // It's in bytes.
          long heapLimitBytes = runtime.maxMemory();
          // Currently allocated heap.
          // It's in bytes
          long allocatedHeapBytes = runtime.totalMemory();
          // Unused memory from the allocated heap.
          // It's an approximation, it's in bytes.
          long unusedAllocatedHeapBytes = runtime.freeMemory();
          // Used memory from the allocated heap.
          // It's an approximation, it's in bytes.
          long usedAllocatedHeapBytes =
               allocatedHeapBytes - unusedAllocatedHeapBytes;
          System.out.println("Max limit for heap allocation: " +
                    getMBytes(heapLimitBytes) + "MB");
          System.out.println("Currently allocated heap: " +
                    getMBytes(allocatedHeapBytes) + "MB");
          System.out.println("Used allocated heap: " +
                    getMBytes(usedAllocatedHeapBytes) + "MB");
     private static long getMBytes(long bytes) {
          return (bytes / 1024L) / 1024L;
}Then I run this program with the option -Xmx1024m, and the result was:
On windows: Max limit for heap allocation: 1016MB
On HP-UX: Max limit for heap allocation: 983MB
Someone knows why the max limit is not 1024MB as I requested?
And why it shows a different value in windows than in HP-UX?
Thanks
Edited by: JoseLuis on Oct 5, 2008 11:29 AM

Answer 1

Thank you for the reply
I have checked and the page size in windows and HP-UX is 4KB.
Also, in the documentation for the -Xmx flag it says that the size must be multiple of 1024 bytes and bigger than 2 MB.
I may understand that the allocated size can be rounded to the nearest page (4 KB block), wich give you a difference less than 1 MB between the requested size and the real allocated size, but in windows the the difference is 8 MB and in HP-UX the difference is 41 MB. It's a big difference.
Am I missing something?