I'm here specifically referring to off-heap memory. I verified this by looking into the jvm with VisualVM. These are objects that aren't part of the normal java heap area and aren't part of garbage collection. The offheap can contain many things - classloaded code, memory-mapped files, direct bytebuffers, and so on. And since it's offheap, it's hard to analyze from Java tooling. So I literally don't know what it was doing.

So in my above scenario, I could indeed run the app with a -Xmx32M and yet the application was still using 200-300MB of resident memory (RSS) due to offheap usage.