Question

Let us think of a memory leak program, wherein a block of heap memory is not freed and the program terminates. If this was (say) a Java Program, the in-built garbage collector would have automatically deallocated this heap block before the program exits.

But even in C++, if the program exits, wouldn’t the Kernel automatically de-allocate all space associated with the process. Also in the Java code, the kernel would have to de-allocate the space for the text part (code) of the process (even if the stack and heap parts are deallocated by the garbage collector). So is the overall advantage of using a garbage collector feature – just the increased savings in time required to deallocate the heap by the program itself rather than the kernel? (if there is any such savings)

EDIT: A primary doubt of mine that has come about looking at the responses – will the GC invoke itself automatically when memory usage reaches a limit? Because, if a GC is only invoked just before the program terminates, it is not going to be useful for long programs.

Answer 1

1. That assumes that the kernel cleans up after you. Not all OSs take care of dynamically-allocated memory automatically. (But to be fair: Most modern ones, at least on the desktop, do.)
2. Even the OSs that reclaim all memory only do so when the process terminates. Most programs allocate far more memory over their total runtime than they need at any given point in time (when run long enough, where “long” can be a few seconds for many data-crunching applications).
3. Because of that, many – especially long-running – processes would create more and more garbage (memory that isn’t used any more, and won’t be used ever again) over their lifetime without any hope of getting rid of it without terminating. You don’t want to kill and restart the whole process just to keep memory usage low, do you?
4. Because unused memory is almost never (there are quite a few processes that run indefinitely and some that can run for hours) disposed of, you get serious memory shortage after a while. Your browser keeps all images, HTML documents, JS objects, etc. you opened during this session in memory because you won’t bother to restart it every few minutes. That’s bullshit and a serious problem in the browser, you say? My point exactly.
5. Moreover, most (that is to say, all good ones) GCs don’t deallocate everything – they run from time to time when they think it’s worth it, but when the process shuts down, everything that remains in memory is left to a lower level (be it a custom allocator or the OS) to be freed. This is also why finalizers aren’t guaranteed to run – in a short-running program that doesn’t make many allocations, the GC may never run.

So no, GC isn’t about saving time. It’s about saving tons of memory, preventing long-running allocation-intense programs from hogging all available memory and eventually making everyone die from out of memory errors.