我从一篇文档上截取下来的几块描述：

1.Further bolstering its reputation as a good open source citizens,Facebook has open sourced an in-house developed tool called watchman,designed to help Web developers speed up build times by automatically tracking file changes and kicking off automated responses.

@H_403_10@2.Facebook's engineering team used Watchman,which runs on Linux,OS X,FreeBSD,and Solaris,to kick off builds while engineers are editing files. "Between this and more intelligent incremental processing,we've been able to reduce our interactive-user p50 build times by 60 percent," Furlong wrote. @H_403_10@"It's not a silver bullet for making builds go fast,but it has helped us to make noticeable improvements both here and in some other internal projects."

总结一下：watchman是个文件监控系统，他优化的是reload文件遍历机制，加入watchman的RN，reload会加快，没加的会遍历量很高，很慢

文章地址：http://www.tuicool.com/articles/Z3UFzu