That's quite platform-dependent, and even the notion of "memory usage" is pretty vague in a modern OS with virtual memory. The size of the malloc heap is usually the most useful, I've found; more generally it's the amount of address space that's unshared and mapped as writeable. Utilities like 'top' or macOS's Activity Monitor can show that as a live value you can monitor.
I'm not sure the Nim APIs above will be as accurate because I think they only report on memory allocated by Nim itself. If you're using any native libraries it probably won't include memory they allocate. If you're on macOS (the only platform I know well), the `heap` tool is very useful. It reports a process's heap usage and breaks it down by the size of the blocks. It can also identify Objective-C, CoreFoundation and [some] C++ objects and group them by class name, which is useful if you're using system frameworks. Check the man page for details.
