小结：
1、
micro-thread with no implicit scheduling; coroutines, in other words.
没有显式调度的微线程，换言之 协程
2、
一个greenlet切换到另一个greenlet，前者被suspend推迟、暂停
uWSGI项目 — uWSGI 2.0 文档 https://uwsgi-docs-zh.readthedocs.io/zh_CN/latest/#
循环引擎 (实现事件和并发，组件可以在reforking, threaded, asynchronous/evented和green thread/coroutine模式下运行。支持多种技术，包括uGreen, Greenlet, Stackless, Gevent, Coro::AnyEvent, Tornado, Goroutines和Fibers)
greenlet: Lightweight concurrent programming — greenlet 0.4.0 documentation
https://greenlet.readthedocs.io/en/latest/#greenlet-lightweight-concurrent-programming
The “greenlet” package is a spin-off of Stackless, a version of CPython that supports micro-threads called “tasklets”. Tasklets run pseudo-concurrently (typically in a single or a few OS-level threads) and are synchronized with data exchanges on “channels”.
A “greenlet”, on the other hand, is a still more primitive notion of micro-thread with no implicit scheduling; coroutines, in other words. This is useful when you want to control exactly when your code runs. You can build custom scheduled micro-threads on top of greenlet; however, it seems that greenlets are useful on their own as a way to make advanced control flow structures. For example, we can recreate generators; the difference with Python’s own generators is that our generators can call nested functions and the nested functions can yield values too. (Additionally, you don’t need a “yield” keyword. See the example in test/test_generator.py).
Greenlets are provided as a C extension module for the regular unmodified interpreter.
greenlet: Lightweight concurrent programming — greenlet 0.4.0 documentation
https://greenlet.readthedocs.io/en/latest/#introduction
A “greenlet” is a small independent pseudo-thread. Think about it as a small stack of frames; the outermost (bottom) frame is the initial function you called, and the innermost frame is the one in which the greenlet is currently paused. You work with greenlets by creating a number of such stacks and jumping execution between them. Jumps are never implicit: a greenlet must choose to jump to another greenlet, which will cause the former to suspend and the latter to resume where it was suspended. Jumping between greenlets is called “switching”.
When you create a greenlet, it gets an initially empty stack; when you first switch to it, it starts to run a specified function, which may call other functions, switch out of the greenlet, etc. When eventually the outermost function finishes its execution, the greenlet’s stack becomes empty again and the greenlet is “dead”. Greenlets can also die of an uncaught exception.