[{"_1":2,"_3":-5,"_4":-5},"loaderData",{"_5":6,"_7":8},"actionData","errors","root",{"_11":-5,"_55":56,"_57":58,"_59":60},"routes/user.$name.replies",{"_9":10,"_11":12,"_13":14,"_15":16,"_17":18,"_19":16},"replies",[35],"user",{"_20":21,"_13":14,"_22":23,"_24":25,"_26":27,"_28":29,"_30":31,"_32":16,"_33":34},"loginname","buddy1","page",1,"limit",50,"total","id","11115","avatar_url","http://www.gravatar.com/avatar/a0edddf67bd00be012b738c3f6125168?size=48","githubUsername","","create_at","2013-11-20T15:23:08.691Z","score",5,"topic_count",0,"reply_count","is_star",false,{"_20":36,"_37":38,"_39":-5,"_40":41,"_42":43,"_44":45,"_46":34,"_47":34,"_32":29,"_48":49,"_26":50,"_51":52},"1504","author_id","4644","tab","content","翻译的一片文章，原文在[这里](http://blog.mixu.net/2011/02/01/understanding-the-node-js-event-loop/)（ps:这个博客我推荐看下，里面还有博主自己写的node.js电子书，质量很高），很基础的东西，个人感觉理解了这个，对以后node开发帮助很大，也不会去做用js手写异步函数的傻事了。但是直接读英文觉得没什么，翻成中文有些地方真觉得罗嗦。。。翻译有问题的地方，欢迎在下面或者[我的博客](http://www.imjustfly.net/article/5044e40bcd8851b110000001)里指出。\r\n\r\n**正文如下：**\r\n\r\n* * *\r\n\r\n关于Node.js的第一个基本概念是I/O操作开销是巨大的：\r\n\r\n![I/O开销](http://ww1.sinaimg.cn/large/a74ecc4cjw1dwjtv0140qj.jpg)\r\n\r\n所以，当前变成技术中最大的浪费来自于等待I/O操作的完成。有几种方法可以解决性能的影响：\r\n\r\n* **同步方式**：按次序一个一个的处理请求。利：简单；弊：任何一个请求都可以阻塞其他所有请求。\r\n* **开启新进程**：每个请求都开启一个新进程。利：简单；弊：大量的链接意味着大量的进程。\r\n* **开启新线程**：每个请求都开启一个新线程。利：简单，而且跟进程比，对系统内核更加友好，因为线程比进程轻的多;弊:不是所有的机器都支持线程，而且对于要处理共享资源的情况，多线程编程会很快变得太过于复杂。\r\n\r\n第二个基本概念是每个连接都创建一个新线程是很消耗内存的（例如：你可以对比Nginx回想一下Apache内存耗尽的情景）。\r\n\r\nApache是多线程的：它为每个请求开启一个新的线程（或者是进程，这取决于你的配置），当并发连接增多时，你可以看看它是怎么一点一点耗尽内存的。Nginx和Node.js不是多线程的，因为线程的消耗太“重”了。它们两个是单线程、基于事件的，这就把处理众多连接所产生的线程/进程消耗给消除了。\r\n\r\n**Node.js中你的代码运行在单线程之中**\r\n\r\n确实只有一个线程：你不能并行执行任何代码，比如：下面的“sleep”将会阻塞sever1秒钟：\r\n\r\n while(new Date().getTime() < now + 1000) {\r\n // do nothing\r\n }\r\n\r\n当这段代码运行时，Node.js不会响应客户端任何请求，因为只有一个线程来运行你的代码，另外，如果你执行cpu密集的任务，比如重设图像的大小，它也会阻塞所有请求。\r\n\r\n**……然而，除了你的代码，其它的一切都是并行执行的**\r\n\r\n单线程没法让代码并行执行。但是所有I/O操作都是事件驱动、并行的，所以下面的代码不会阻塞server：\r\n\r\n c.query(\r\n 'SELECT SLEEP(20);',\r\n function (err, results, fields) {\r\n if (err) {\r\n throw err;\r\n }\r\n res.writeHead(200, {'Content-Type': 'text/html'});\r\n res.end('Hello

Return from async DB query

');\r\n c.end();\r\n }\r\n );\r\n\r\n如果你在一次请求中执行这些，当数据库sleep时，其他请求也会立即被处理。\r\n\r\n**为什么异步比较好？什么时候我们应该从同步转移到异步/并行执行呢？**\r\n\r\n同步执行也不错，因为它简便了我们敲代码。但在使用异步时，你不必关心后端是怎么处理的。而且，在I/O操作时不会阻止其他请求，同时无需承担每个请求所产生的线程/进程的成本。\r\n\r\nI/O操作时使用异步处理很好，因为I/O操作的成本比单纯执行代码要高的多，我们应该在等待I/O时做其它更有意义的工作。\r\n\r\n![Node.js event loop](http://ww3.sinaimg.cn/large/a74eed94jw1dwjtwpu7sjg.gif)\r\n\r\nEvent loop是指处理外部事件，并把外部事件转换为回调来进行调用的实体（晦涩难懂！！原文：_an entity that handles and processes external events and converts them into callback invocations_）.所以，I/O调用的同时，server就可以去处理另一个请求。在一次I/O调用中，你的代码保存回调函数并把控制权交回到node.js运行时。当数据加载完毕可以访问时，就可以执行回调函数了。\r\n\r\n当然，在后端有很多数据库接入和处理的进程。但是，这些都不需要通过你的代码直接实现，你也就不必了解后台I/O之间的相互作用关系。和Apache相比，省去了很多线程消耗，因为不是每个链接都需要新线程，仅那些需要并行运行的才需要新线程。\r\n\r\n不只是I/O调用，Node.js期望所有的请求都能快速的响应，比如CPU密集的工作应该分离到其他进程，你可以使用事件和他交互。\r\n\r\n**内部实现**\r\n\r\n在内部，Node.js依赖libev来提供event loop，使用线程池来提供异步I/O。","title","理解Node.js的event loop","last_reply_at","2013-12-10T13:49:34.620Z","good","top","visit_count",21761,"2012-09-04T16:41:53.196Z","author",{"_13":53,"_22":54},"justfly","http://www.gravatar.com/avatar/9079fee9c57e939ffc1d500c156b831d?size=48","zones",[106],"tabs",[74,75,76,77,78,79],"publicConfig",{"_61":62,"_63":25,"_64":65},"apiBaseUrl","https://api.cnodejs.org","turnstileSiteKey","build",{"_66":67,"_68":69,"_70":71,"_72":73},"service","cnode-web","version","0.1.0","commit","09940aa1bc0ad3dfe64235540ced544300ba2865","buildTime","2026-08-02T04:10:22Z",{"_20":16,"_81":104,"_83":105,"_85":96,"_86":16,"_88":97},{"_20":101,"_81":102,"_83":103,"_85":96,"_86":101,"_88":97},{"_20":98,"_81":99,"_83":100,"_85":96,"_86":98,"_88":97},{"_20":94,"_81":46,"_83":95,"_85":96,"_86":94,"_88":97},{"_20":90,"_81":91,"_83":92,"_85":34,"_86":93,"_88":89},{"_20":80,"_81":82,"_83":84,"_85":34,"_86":87,"_88":89},10,"key","test","label","测试","visible","sort_order",91,"scope","admin",9,"dev","开发",90,4,"精华",true,"public",3,"job","招聘",2,"ask","问答","share","分享",{"_20":16,"_107":108,"_109":100,"_110":111,"_112":113,"_85":96,"_86":16},"slug","jobs","name","description","Node.js 招聘信息专区","icon","briefcase"]