VirtualBox host, memory management issue, unable to release unused memory of guest

[stl0hello]

I have a workstation with 32 GB installed memory.

I create a new Debian virtual machine, and allocate 16 GB to it. When I boot the machine, it only take less than 800 MB memory. As the virtual machine running, especially for heavy tasks. The memory occupation increase. But when I end those tasks, the memory usage of guest machine's OS suddenly reduce, but the memory occupation of virtual machine on host RAM, still keep as the memory upper bound.

I believe that it is the problem of VirtualBox, because even if I reboot (not same as power off and boot) the virtual machine, the RAM usage still keep as Maximum.

I hope VirtualBox can add ability to automatically release unused guest memory.

[yetanothervboxuser]

This thread seems to be related ​https://forums.virtualbox.org/viewtopic.php?f=8&t=97181