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What is IO multiplexing

本章讨论"What is IO multiplexing"，重要参考的内容有:

一、zhihu I/O多路复用技术（multiplexing）是什么？

收录在 zhihu-What-is-IO-multiplexing 章节中，通过这篇文章，基本上了解了Linux IO multiplexing。需要注意的是，这篇文章所讲述的是Linux的implementation，它并不能够代表IO multiplexing的全部。

二、APUE 14.4 I/O Multiplexing

三、unp Chapter 6. I/O Multiplexing: The select and poll Functions

四、wiki Io Multiplexing

I/O multiplexing means what it says - allowing the programmer to examine and block on multiple I/O streams (or other "synchronizing" events), being notified whenever any one of the streams is active so that it can process data on that stream.

In the Unix world, it's called select() or poll() (when using the CeeLanguage API for Unix). In the

MicrosoftWindowsApi world, it's called WaitForMultipleObjects(). Other languages/environments have similar features:

The advantage of IoMultiplexing is that it allows blocking on multiple resources simultaneously, without needing to use polling (which wastes CPU cycles) or multithreading (which can be difficult to deal with, especially if threads are introduced into an otherwise sequential app only for the purpose of pending on multiple descriptors).

With the understanding, of course, that the CPU cycles are gonna get burned somewhere anyway, and even if your task/process is not threaded, the system as a whole will have other threads/tasks running.

IO multiplexing是一种event notification mechansim

TODO

Multiplexing and demultiplexing

在 What-is-multiplexing 章节中，我们知道了:

如何来理解multiplexing？multiplexing的关键在于many和one，下面是一些例子:

在 computing的 **I/O multiplexing**中，many是multiple input/output events 而one则是a single event loop object；

many IO event and one event loop object；

在现实application中，都需要有reverse operation: demultiplexing，**demultiplexing**其实非常类似于dispatch

在 IO multiplexing mechansim中，同样有着 "demultiplexing" 的存在，关于此的文章有:

1、zhihu 如何深刻理解reactor和proactor？

Event Demultiplexer and Event Handler

一般地,I/O多路复用机制都依赖于一个事件**多路分离器(Event Demultiplexer)。**分离器对象**可将来自事件源的I/O事件分离出来，并分发到对应的**read/write事件处理器(Event Handler)(或回调函数)。开发人员预先注册需要处理的事件及其事件处理器（或回调函数）；事件分离器负责将请求事件传递给事件处理器。

Use pattern: reactor 和 proactor

由于reactor 和 proactor一般归入design pattern的范畴，因此，将它们放到了工程parallel-computing中了。

两种pattern都使用了IO multiplexing，但是它们之间有着差别，通过它们的差别我们能够看出IO multiplexing的本质，我们也知道，Linux IO multiplexing是使用reactor的。

关于此的非常好的文章:

1、zhihu 如何深刻理解reactor和proactor？