MongoDB will (at least seem) to use up a lot of available memory, but it actually leaves it up to the OS's VMM to tell it to release the memory (see Caching in the MongoDB docs.)
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Depending on the platform you may see the mapped files as memory in the process (see the Virtual Memory section below), but this is not strictly correct. Unix top may show way more memory for mongod than is really appropriate for what is in physical memory. The Operating System (the virtual memory manager specifically, depending on OS) manages the memory where the "Memory Mapped Files" reside. This number is usually shown in a program like "free -lmt".