Mastering Rust - Second Edition

User-level

Process- and thread-based concurrency are limited by how many of them we can spawn. A lighter and more efficient alternative is to use user space threads, popularly known as green threads. They first appeared in Java with the code name green and the name has stuck since then. Other languages such as Go (goroutines), and Erlang also have green threads. The primary motivation in using green threads is to reduce the overhead that comes with using process- and thread-based concurrency. Green threads are very lightweight to spawn and use less space than a thread. For instance, in Go, a goroutine takes only 4 KiB of space compared to the usual 8MB by a thread.

User space threads are managed and scheduled as part of the language runtime. ...