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I'm relatively new to Java programming so please bear with me trying to understand what's going on here.

The application I've developed uses a max heap size of 256MB. With the GC being done, I never run into any problems with this. The used heap builds up when a big image is loaded and gets freed nicely when it is unloaded. Out of memory errors are something that I've yet to see.

However... Running the application for about an hour. I notice that the process uses more and more system memory that never gets freed. So the application starts with around 160MB used, builds up as the heap size grows, but when the heap size shrinks the system memory used just keeps getting more. Up until the process uses 2.5GB and my system starts to become slow.

Now I'm trying to understand the surviving generations bit. It seems the heap size and surviving generations aren't really connected to each other? My application builds up a lot of surviving generations, but I never run out of memory according to the used memory by the application itself. But the JVM keeps eating memory, never giving it back to the system.

I've searching around the web, sometimes finding information that is somewhat useful. But what I don't get is that the application stays well within the heap size boundaries and still my system memory is being eaten up.

What is going on here?

I'm using NetBeans IDE on OSX Lion with the latest 1.6 JDK available.

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  • Did you try poking at your app with VisualVM? It should have a tool to look at a heap dump, maybe knowing what objects are taking up the old generations would help. Commented Nov 30, 2011 at 20:56
  • Are you sure you are properly disposing of all resources? Certain classes (like ImageReader for example) can hold onto some native resources, which are not automatically released when the object itself is collected in the JVM. Usually you need to use finally-blocks to ensure that such resources are released. Commented Nov 30, 2011 at 20:59
  • So you have a -Xmx of 256m but the process size grows to 2.5gb? Wow. I've never seen that sort of behavior. How many threads are you using? Commented Nov 30, 2011 at 21:27

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The best way to start would be jvisualvm from the JDK on the same machine. Attach to your running program and enable profiling.

Another option is to try running the application in debug mode and stop it once in a while to inspect your data structures. This sounds like a broken/weird practice but usually if you have a memory leak it becomes very obvious where it is.

Good luck!

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I've been profiling with the NetBeans profiler, which actually does somewhat the same as jvisualvm. The heap size on a Windows PC stays well below 64MB while the heap size isn't limited. On OSX the heap size grows to a maximum of 128MB now. However... The memory used by the system still keeps growing. There are a few surviving generations according to the profiler, the surviving generations in the graph just keep growing none the less. It's a dynamic application. Objects get created and thrown away constantly.

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