When Watchman is not installed, a notice is displayed when invoking various commands. You can safely ignore this message:

$ ember version
Could not start watchman; falling back to NodeWatcher for file system events.
Visit http://ember-cli.com/user-guide/#watchman for more info.

Here is what Watchman is and why you should not ignore this message.

What is Watchman?

Watchman is a simple utility to watch files/directories and perform an action when they change. Watchman provides Ember CLI a more effective way for watching project changes. On OSX and UNIX-like operating systems, Ember recommend installing Watchman version 3.x.

Why not let Ember CLI fallback to NodeWatcher?

File-watching on OSX is error-prone and Node’s built-in NodeWatcher has trouble observing large trees. Watchman on the other hand, solves these problems and performs well on extremely massive file trees.

How do I use Watchman instead of NodeWatcher?

On OSX, you can install Watchman using Homebrew:

$ brew install watchman