For a recent Facebook hackathon I submitted a pull request to the Watchman project adding a gem for easily talking to Watchman from Ruby using the high-performance binary protocol. This is code written in C for speed, which I had prepared for Command-T, so the job here was to extract it and package it into an easy-to-use format.
Watchman is somewhat of a hidden treasure. It was a key part in making Command-T pleasant to use with extremely large source code repos (of the order of half-a-million files, like Google’s Chrome codebase). In the worst case scenario (cold file-system cache), scanning such a hierarchy can take multiple minutes, even when using a fast C-based scanner like the
find executable. With Watchman, the typical "scan" time drops to under a second, meaning that you can open and close your Vim instances without having to worry about a lengthy scan the first time you use Command-T.