Links die. So quoting off the mongodb manual
Not necessarily. It’s certainly possible to run MongoDB on a machine with a small amount of free RAM.
MongoDB automatically uses all free memory on the machine as its cache. System resource monitors show that MongoDB uses a lot of memory, but it’s usage is dynamic. If another process suddenly needs half the server’s RAM, MongoDB will yield cached memory to the other process.
Technically, the operating system’s virtual memory subsystem manages MongoDB’s memory. This means that MongoDB will use as much free memory as it can, swapping to disk as needed. Deployments with enough memory to fit the application’s working data set in RAM will achieve the best performance.