By default ctrlp will look for the root of your repo (by looking for a .git, .hg, .svn or .bzr file) and then start searching for files in that folder. For the project I’m currently working on that has too many files (Probably millions) I prefer that ctrlp only searches inside the folder were I started vim. This can be done with this setting in .vimrc:

let g:ctrlp_working_path_mode = 'a'
[ productivity  vim  ]
Vim command line mode
The Vim statusline
Spell and Grammar checking on vim
Disable expandtab in Vim
Adjusting the width of Nerdtree navigator