What do -Xms and -Xmx do?

Question

I know what they are supposed to do, but am unable to see any tangible results from setting them...

Answer 1

On my machine they do exactly what they're supposed to...that is, to set the initial and max memory available for a program. I wrote two programs to test this :
  //this shows how much memory is available at start up
  public void main(String args[]) {
    System.out.println(Runtime.getRuntime().freeMemory());
  //this outputs how many KB memory the program sucked before crashing.
  public static void main(String args[]) throws Exception {
    Vector vec = new Vector();
    int i = 0;
    try {
      while (true) {
        vec.add(new byte[1024]);
        ++i;
    } finally {
      System.out.println(i);
    }When I run the first program w/o -x set, I get around 1.5 mb (1760896 bytes); when I invoke it with -Xms16m it returns 16 mb (16440960 bytes). The second program crashes at around 64mb (63633 to be exact). Invoking it with -Xmx16m crashes it at 16 mb, as expected (15813).
Both -Xmx and -Xms are unsupported options...thus their effects are implementation-dependent. It's thus possible that your implementation doesn't support them. (I'm running this on Win2000 using Sun's JVM 1.4.1_01-b01.)
The only other thought that occurs to me is that you're sending the wrong parameters...-Xms16 means you want to set the initial stack to 16 bytes, obviously something that would have little effect. -Xms16k would be 16kbytes, -Xms16m is in megabytes.