If/When does the does deallocated heap memory get reclaimed?

Question

I have been running overnight memory tests on an embedded Linux system. Using vmstat I have observed that the free memory steadily decreases over time. According to some

Answer 1

Calling delete in your program causes memory to return to the memory manager that is part of your program runtime. In principle this could be written so as to return freed memory to the OS, but I would be surprised if it did. Rather the recycled memory is kept aside for subsequent calls to new.

Note that this is the virtual memory of your process; how much of it is actually residing in physical memory at any time during program execution depends on overall system load and is handled by the operating system.