1 多进程、多线程和协程
python并发编程之多进程、多线程、异步和协程：http://www.cnblogs.com/tyomcat/p/5486827.html
1.1 总结
1）多进程能够利用多核优势，但是进程间通信比较麻烦，另外，进程数目的增加会使性能下降，进程切换的成本较高。程序流程复杂度相对I/O多路复用要低。
2）I/O多路复用是在一个进程内部处理多个逻辑流程，不用进行进程切换，性能较高，另外流程间共享信息简单。但是无法利用多核优势，另外，程序流程被事件处理切割成一个个小块，程序比较复杂，难于理解。
3）线程运行在一个进程内部，由操作系统调度，切换成本较低，另外，他们共享进程的虚拟地址空间，线程间共享信息简单。但是线程安全问题导致线程学习曲线陡峭，而且易出错。
4）协程有编程语言提供，由程序员控制进行切换，所以没有线程安全问题，可以用来处理状态机，并发请求等。但是无法利用多核优势。
1.2 协程
介绍：http://blog.csdn.net/k_zombie/article/details/50839961协程不同于线程的地方在于协程不是操作系统进行切换，而是由程序员编码进行切换的，也就是说切换是由程序员控制的，这样就没有了线程所谓的安全问题。
协程的缺点：无法利用多核优势，不过，这个可以通过协程+进程的方式来解决。如果你的任务类型不是那种cpu密集的，那选用协程是个好选择。
1.3 多线程
参考：http://blog.csdn.net/k_zombie/article/details/50769128
2 python yield和generator
参考：http://blog.csdn.net/k_zombie/article/details/50815256
3 greenlet
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 tests/test_generator.py.
Greenlets are provided as a C extension module for the regular unmodified interpreter.
Greenlets are lightweight coroutines for in-process concurrent programming.
Who is using Greenlet?
There are several libraries that use Greenlet as a more flexible alternative to Python’s built in coroutine support:
Getting Greenlet
The easiest way to get Greenlet is to install it with pip or easy_install:
pip install greenlet
easy_install greenlet
Source code archives and windows installers are available on the python package index at https://pypi.python.org/pypi/greenlet
The source code repository is hosted on github: https://github.com/python-greenlet/greenlet
Documentation is available on readthedocs.org: https://greenlet.readthedocs.io
2.2 源码分析
python协程的实现（greenlet源码分析）：http://blog.csdn.net/fjslovejhl/article/details/38824963
python协程入门（greenlet）：http://blog.csdn.net/fjslovejhl/article/details/38821673
python greenlet背景介绍与实现机制：http://blog.csdn.net/offbye/article/details/39368781
2.3
3 Eventlet
官网：http://eventlet.net/Eventlet is a concurrent networking library for Python that allows you to change how you run your code, not how you write it.
- It uses epoll or kqueue or libevent for highly scalable non-blocking I/O.
- Coroutines ensure that the developer uses a blocking style of programming that is similar to threading, but provide the benefits of non-blocking I/O.
- The event dispatch is implicit, which means you can easily use Eventlet from the Python interpreter, or as a small part of a larger application.
eventlet是一个用来处理和网络相关的Python库函数，而且可以通过协程来实现并发，在eventlet里，把“协程”叫做greenthread(绿色线程)。
参考： openstack nova 基础知识——eventlet：http://blog.csdn.net/xiangmin2587/article/details/8182775
Openstack Eventlet分析(1)：http://blog.csdn.net/gaoxingnengjisuan/article/details/12913275
Openstack Eventlet分析(2)：http://blog.csdn.net/gaoxingnengjisuan/article/details/12914831