在Medium上看到一个非常棒的用来描述各种IO模型的伪代码，这些例子很好地展示了阻塞非阻塞、同步和异步的特点。

常见的I/O 类型

以读操作为例，不同的IO模型。对象和函数都是抽象的：
Following examples shows concepts of three I/O approaches on the reading operation. Objects and functions are abstract.

1. 阻塞，同步 Blocking, synchronous:

device = IO.open()
data = device.read() # thread will be blocked until there is data in the device
print(data)

2. 非阻塞，同步 Non-blocking, synchronous:

device = IO.open()
ready = False
while not ready:
    print("There is no data to read!")
    ready = IO.poll(device, IO.INPUT, 5) # returns control if 5 seconds have elapsed or there is data to read (INPUT)
data = device.read()
print(data)

3. 非阻塞，异步 Non-blocking, asynchronous:

ios = IO.IOService()
device = IO.open(ios)

def inputHandler(data, err):
    "Input data handler"
    if not err:
        print(data)

device.readSome(inputHandler)
ios.loop() # wait till all operations have been completed and call all appropriate handlers

4. Reactor 模式 Reactor Pattern

device = IO.open()
reactor = IO.Reactor()

def inputHandler(data):
    "Input data handler"
    print(data)
    reactor.stop()

reactor.addHandler(inputHandler, device, IO.INPUT)
reactor.run() # run reactor, which handles events and calls appropriate handlers

这些几个伪代码例子，非常好地帮助理解不同的IO模型。

为啥会有这些不同的IO模型呢？我个人的理解是，因为IO是一个比较重的耗时操作，为了最大化利用CPU资源，才有这些不同模型。其本质是在极限利用CPU，增加吞吐量和缩减响应时间。

（完）