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javamemory-managementmemory-leaksgarbage-collection

Burst memory usage in Java

I am trying to get a handle on proper memory usage and garbage collection in Java. I'm not a novice programmer by any means, but it always seems to me that once Java touches some memory, it will never be released for other applications to use. In that case, you have to make sure your peak memory is never too high, or your application will continually use whatever the peak memory usage was.

I wrote a small sample program trying to demonstrate this. It basically has 4 buttons...

  1. Fill class scope variable BigList = new ArrayList<string>() with about 25,000,000 long string items.
  2. Call BigList.clear()
  3. Reallocate the list - BigList = new ArrayList<string>() again (to shrink the list size)
  4. A call to System.gc() - Yes, I know this doesn't mean that GC will really run, but it's what we have.

So next I did some testing on Windows, Linux, and Mac OS while using the default task monitors to check on the processes reported memory usage. Here is what I found...

  • Windows - Pumping the list, calling clear, and then calling GC several times will not reduce memory usage at all. However, reallocating the list using new and then calling GC several times will reduce the memory usage back to starting levels. IMO, this is acceptable.
  • Linux (I used Mint 11 distro with Sun JVM) - Same results as Windows.
  • Mac OS - I followed the sames steps as above, but even when reinitializing the list calls to GC seemingly have no effect. The program will sit using hundreds of MB of RAM even though I have nothing in memory.

Can anyone explain this to me? Some people have told me some stuff about "heap" memory, but I still don't fully understand it and I'm not sure it applies here. From what I have heard about it, I shouldn't be seeing the behavior I am on Windows and Linux anyways.

Is this just a difference in the way Mac OS's Activity Monitor measures memory usage or is there something else going on? I would prefer to not have my program idling with tons of RAM usage. Thanks for your insight.

Solution

  • The Sun/Oracle JVM does not return unneeded memory to the system. If you give it a large, maximum heap size, and you actually use that heap space at some point, the JVM won't give it back to the OS for other uses. Other JVMs will do that (JRockit used to, but I don't think it does any more).

    So, for Oracles JVM you need to tune your app and your system for peak usage, that's just how it works. If the memory that you're using can be managed with byte arrays (such as working with images or something), then you can use mapped byte buffers instead of Java byte arrays. Mapped byte buffers are taken straight from the system, and are not part of the heap. When you free up these objects (AND they are GC'd, I believe, but not sure), the memory will be returned to the system. You'll likely have to play with that one assuming it's even applicable at all.