mongodb consumes memory

Configuring 24G memory
The memory requirements for mongodb3.0 are not high.
When the memory is upgraded to 3.2.9, the memory used is a little more, reaching about 80%. mongodb often hangs due to
oom . When the memory is increased to 3.4.5, the memory drops a little bit Maintain at around 40%

Upgrade : From 3.0 to 3.4, you must first upgrade from 3.0 to 3.2, and then from 3.2 to 3.4