In configurations with microsharding it's possible for different mongod's to compete for memory, so cgroups should be used to limit the RAM consumption per mongod process (otherwise processes will affect each other in all the unpredictable ways).

Since the cgroups configuration is a bit non-trivial and is available only to *nix admins with black belt and higher, I think our users will highly appreciate if we provide a short example of setting it up.