There is no single way to control the memory usage using system tools except maintaining mongod instances on dedicated virtual machines without running further services. There are numerous complaints from people about this stupid architectural decision from various side and 10gen is doing nothing to change this brain-dead memory model.
Can someone explain to me why this is actually a big issue? Except for really tiny apps, I imagine that having dedicated VMs for your MongoDB actually would be perfectly fine? Probably even preferred?