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node.js异步:原理和缺陷赵成@小型笨蛋
node.jsutilhttphttpsfsJavascript模块......bufferchild_processfilenetC模块......V8解析器事件库uvlibeiolibev
主要内容异步接口的实现uv,libev,以及libeio存在的问题
异步的实现方式Synchronous I/O Multiplexingselect,pselect,poll,epoll,kqueue,libev线程模拟glibcaio,libeioKernel Native AIO,以及Windows Overlapped I/O前者问题多多,比如仅支持 O_DIRECT 方式来对磁盘读写后者用于在uv中实现Windows的异步I/O在Linux下,node.js靠libev和libeio配合使用来实现异步I/O
事件驱动的一个例子var net = require('net');var server = net.createServer(function (socket) {socket.write("Echo server\r\n");socket.pipe(socket);});server.listen(1337, "127.0.0.1");static Handle<Value> Connect(constArguments&) { ...uv_tcp_connect(..., AfterConnect); ...}
uvThis is the new networking layer for Node. Its purpose is to abstract IOCP on windows and libev on Unix systems. We intend to eventually contain all platform differences in this libraryuv是node的网络实现层
*nix下uv是对libev的封装
主要目的是实现Windows版本的node什么是libevlibev是一个事件驱动库,提供高性能事件循环
主要用于事件驱动的网络编程调用流程Server.listen (1337); listener();TCPWrap::Listen (arguments);OnConnection();listen(tcp->fd, backlog);ev_io_set(..., fd, EV_READ);accept(...);connection_cb();返回有新连接主循环主线程
另一个例子:fs.close(fd)static Handle<Value> Close(const Arguments& args) {intfd = args[0]->Int32Value();if (args[1]->IsFunction()) {ASYNC_CALL(close, args[1], fd)}}#define ASYNC_CALL(func, callback, ...) \eio_##func(After, cb_persist(callback));
什么是libeio Libeio is a full-featured asynchronous I/O library for C, modelled in similar style and spirit as libev. Features include: asynchronous read, write, open, close, stat, unlink, fdatasync, mknod, readdir etc.libeio为C提供异步版本的POSIX API
主要提供文件I/O操作
异步操作通过线程实现
libeio仅依赖pthread,跨平台能力非常好
可以和任何事件库配合使用,比如libevlibeio做了什么fs.close (fd, callback);callback ();Close (arguments);After ();close结束后eio_poll ();eio_close (fd, ..., After);主线程close (fd);新线程
为什么不用libev实现异步文件操作?@爱多对于Regular File 来说,是不能够用采用 poll/epoll的,即O_NOBLOCK 方式对于传统文件句柄是无效的,也就是说我们的 open ,read, mkdir之类的Regular File操作必定会导致阻塞
BAD CASEfunction onFileB(err) {fs.readFile("c");}function onFileA(err, data) { data += "blabla";fs.writeFile ("b", data, onFileB);}fs.readFile ("a", onFileA);
调用过程主线程pollfs.readFileonFileAfs.readFilepollonFileB新线程readread
代价代码变得非常不直观每个fs.readFile都发起一个线程线程间context switch时代价很大如果传递的是匿名函数,那么在执行前,函数的context会一直保存在内存中回调函数执行前资源(比如说数据库连接)不会被释放
BAD CASE 2var conn = db.connect(...);conn.query(‘SELECT * FROM table’, function(err) { ...conn.close();});conn.close();只能在主线程中执行