在了解web服务器及其工作方式时，您可能会好奇，他们是在不断地侦听请求，还是等到收到请求后才采取行动。有鉴于此，今天的超级用户问答帖子有了满足读者好奇心的答案。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

截图由xmodulo/Linux截图（Flickr）提供。

问题

超级用户读者user2202911想知道web服务器如何侦听新请求：

I am trying to understand the ‘deeper level’ details of how web servers work. I want to know if a server, say Apache, for instance, is continuously polling for new requests or if it works by some sort of interrupt system. If it is an interrupt, what is sparking the interrupt? Is it the network card driver?

web服务器如何侦听新请求？

答案

超级用户贡献者Greg Bowser为我们提供了答案：

The short answer is some sort of interrupt system. Essentially, they use blocking I/O, meaning they sleep (block) while waiting for new data.

1. The server creates a listening socket and then blocks while waiting for new connecti***. During this time, the kernel puts the process into an interruptible sleep state and runs other processes. This is an important point; having the process poll continuously would waste CPU resources. The kernel is able to use the system resources more efficiently by blocking the process until there is work for it to do.
2. When new data arrives on the network, the network card issues an interrupt.
3. Seeing that there is an interrupt from the network card, the kernel, via the network card driver, reads the new data from the network card and stores it in memory. (This must be done quickly and is generally handled inside the interrupt handler.)
4. The kernel processes the newly arrived data and associates it with a socket. A process that is blocking on that socket will be marked runnable, meaning that it is now eligible to run. It does not necessarily run immediately (the kernel may decide to run other processes still).
5. At its leisure, the kernel will wake up the blocked web server process. (Since it is now runnable.)
6. The web server process continues executing as if no time has passed. Its blocking system call returns and it processes any new data. Then go to step 1.

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