heap memory is not released to system

[X512]

This is hrev54155.

Test program: https://dev.haiku-os.org/attachment/ticket/15995/MemTest.cpp.

Steps to reproduce:

1. Enable ALLOC_MALLOC and set allocSize to 64.
2. Run program until totalSize: message will be displayed.
3. Press enter and wait until totalSize: 0 message will be displayed.
4. Memory is still allocated by test program that can be confirmed by ProcessController or ActivityMonitor.
5. Press enter again that will cause program exit and freeing memory.

[pulkomandy]

This is expected. The team preserves its heap unless the memory is needed elsewhere (the low_resource_manager would request it to release things). Releasing pages to the system pool and then reclaiming them when a new allocation happens would waste a lot of performance.

[X512]

Memory is not released when requested by another process, it just fail. You can run another instance of test program to confirm that.

[waddlesplash]

Yes, this is because the malloc we use (hoard2) is essentially sbrk-based, so it can only return memory when it can resize/cut the area in a sane way. This should be fixed when we switch to a different allocator that uses mmap() instead.