Question

I have a bunch of classes that i have to tweak for improved memory usage in exchange for CPU time, and i have junit 3 as a test framework to prove it.

While i can easily do that (use string.getBytes("UTF-8"); stuff multiple booleans in a byte, use long instead of Date, implement my own collections etc), I don’t really have a solid way to show improvements when i commit the code, because of the impredictable nature of the Garbage Collector.

When i am talking about collecting garbage, i mean i have done both of these 2 things: called System.gc(), AND tried to allocate an array like so long[] longs=new long[Integer.MAX_VALUE];
These are a few algorythms I tried to implement without success:

1:

a:garbage collect;

b:get the ammount of memory used at this moment; (long beforeold=Runtime.getRuntime().maxMemory - Runtime.getRuntime().freeMemory(); )

c:allocate an array of a few tens of thousands of instances of my old class;

d:garbage collect;

f: long afterold=Runtime.getRuntime().maxMemory - Runtime.getRuntime().freeMemory();

g:repeat the process instantiating the new classes

h:compare the differences in memory.

The results i got were freaking weird:
My “afterold” was less than my “beforeold” indicating some kind of weird garbage collecting event occured during memory allocation for the new classes.

The memory used by the new class instance turned out results anywhere from negative, to positive, signaling again some weird garbage collecting in the middle of instantiation.

Now, has anyone else tried to prove some classes are more efficient than others like this? It’s a very cool problem, but it’s still a problem.

Thx, you guys rule!

Answer 1

Heinz Kabutz has some tips in Java Specialists newsletter: http://www.javaspecialists.eu/archive/Issue193.html:

public long calculateMemoryUsage(ObjectFactory factory) {
  Object handle = factory.makeObject();
  long memory = usedMemory();
  handle = null;
  lotsOfGC();
  memory = usedMemory();
  handle = factory.makeObject();
  lotsOfGC();
  return usedMemory() - memory;
}

private long usedMemory() {
  return Runtime.getRuntime().totalMemory() -
      Runtime.getRuntime().freeMemory();
}

private void lotsOfGC() {
  for (int i = 0; i < 20; i++) {
    System.gc();
    try {
      Thread.sleep(100);
    } catch (InterruptedException e) {
      Thread.currentThread().interrupt();
    }
  }
}

public void showMemoryUsage(ObjectFactory factory) {
  long mem = calculateMemoryUsage(factory);
  System.out.println(
      factory.getClass().getSimpleName() + " produced " +
          factory.makeObject().getClass().getSimpleName() +
          " which took " + mem + " bytes");
}