Force memory release to the OS

Question

I have an app that takes about 20 MB of ram. In an seldom used algorithm it (std::vector) temporarily allocates 250 MB. After the deallocation the systemmonitor still shows

Answer 1

You can't, and shouldn't.

Virtual memory allocation is complicated, and cannot be sufficiently understood by simply watching a number in System Monitor. It may appear as if a process is using more memory than it should, but this is just an artefact of the way virtual memory addressing works.

Rest assured, if you have freed this memory properly, and the OS really needed it back, it would be reassigned.

The only real actionable point here is to stop using System Monitor as if it were an accurate measure of physical RAM in use by your process!

Answer 2

Use mmap() or VirtualAlloc() to allocate and release memory. This returns it to the OS immediately.

In order to use with std::vector, you'll need to provide it a std::allocator. You might find it easier to hand-roll your own vector w/ placement new and direct destructor invocation.

Normally the system heap allocators handle this correctly for you; however it looks like you found a case where they do not.