We're commonly running into this warning in environments we feel are properly configured. It's a warning, not an error, but it's still creating friction through confusion among the user base, so it would be nice to address.

WARNING: soft rlimits too low. rlimits set to XXX processes, YYY files. Number of processes should be at least YYY/2 : 0.5 times number of files.

The issue is that WiredTiger now uses a file descriptor per namespace, meaning the number of file descriptors required can quickly outstrip the number of connections used in heavily multi-tenanted environments (commonly development and test) where connection requirements are modest but namespaces abound.

You can see a more detailed discussion from the mongodb-user group:

https://groups.google.com/d/topic/mongodb-user/aUrWTZxUUi4/discussion