I'm writing a simple malloc implementation for a college project. One of the tasks is to sometimes give back freed memory to the OS (the example given was of a process using say 1GB malloc-ed memory during a period, and afterwards it only uses 100MB memory until it terminates), however I'm not sure how to implement this. I was thinking of periodically checking the amount of memory the process has allocated and the amount freed and, if possible, give back some of the freed pages to the OS, but I'm not sure if this is an efficient approach.
EDIT: I didn't realize when I first wrote this, but the way I worded this is too vague. By "unused memory" I'm talking specifically about freed one.
Asking the OS for memory or returning it back are (relatively) expensive operation because they require a context switch user/kernel and back. For that reason, in most implementations, the malloc call only asks for large chunks and internally allocates from those chunks, and manages freed memory with a free blocks list. In that case, it only returns memory to the OS when a full chunk is present in the free list.
For a custom implementation, the rule for returning memory to the system is up to the programmer (you...).