You might start mongo demon as
but, as you close the terminal this will terminate your session.
So, to start it as a service so that it would continue running
mongod --fork --logpath /path/to/log/file
You can specify the log file as you wish
This is typical memory issue. Are you running a bunch of heavy programs along side? Check how much pages you are loading on chrome even.. and your IDE..
There is a way to add more before you start mongod though
ulimit -n 2000 && mongod
But I am a fan of using the resource effectively by removing apps that takes up my memory..