delphi - How to convince the memory manager to release unused memory

Question

In a recent post ( My program never releases the memory back. Why? ) I show that when using FastMM, the application does not release substantial amounts of memory back to the system. Recently I created an artificial test program to make sure the issue it is not a memory and that it only appears with FastMM.

In this program I create and destroy an object (same as the one used in the previous post) 500 times.

The memory requirements are ("Private working set"):

Without FastMM
Before running the loop: 1.2MB
After running the loop: 2.1MB

With FastMM (aggressive debug mode)
Before running the loop: 2.1MB
After running the loop: 25MB

With FastMM (release mode)
Before running the loop: 1.8MB
After running the loop: 3MB

If I run the loop several times, the memory requirement does not increase. Which means that the unreleased memory is re-used so this is not a memory leak (a memory leak would increase the memory footprint with several KB/MB at each run).

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My questions are:

How can I disable this behavior in FastMM? Is it even possible? I know, if I release the program without FastMM or with FastMM Release Mode it will "waste" moderate amounts of RAM. But disabling this behavior on demand, will help me (us?) identifying memory leaks. Actually in my first post (see link) many people suggested that I have a leak. The confusion was created obviously just because of this behavior. No, it is obvious there is no leak. It is just the memory manager that refuses to release large amounts of memory.

It will ever release the extra memory? When? What triggers this? Can the programmer trigger it? For example when I know that I have finished a RAM-intensive task and the user may not use the program for a while (minimize it), can I flush the RAM back to the system? What happens when the user open multiple instances of my program? Won't they compete for RAM?

Answer 1

You shouldn't think about it as "wasting" RAM, really. Think about it as "caching" unused RAM. The memory manager is holding onto the unused memory instead of releasing it back to the OS for a reason, and in fact you've hit upon that reason in your question.

You said that you keep re-running the same operations in a loop. When you do that, it still has the old memory available and it can assign it immediately, instead of having to ask Windows for a new chunk of heap. This is one of the tricks that puts the "Fast" in "FastMM," and if it didn't do that you'd find your program running a lot more slowly.

You don't need to worry about the FastMM debug mode figure. That's only for debugging, and you're not going to release a program compiled against FullDebugMode. And the difference between "without FastMM" and "with FastMM Release Mode" is about 1 MB, which is negligible on modern hardware. For the low cost of only 1 extra MB, you get a big performance boost. So don't worry about it.