深入理解 tornado 之 底层 ioloop 实现（二）

def configurable_default(cls): """Returns the implementation class to be used if none is configured.""" raise NotImplementedError() 

显然也是个接口，那么我们再回头看 ioloop 的 configurable_default():

def configurable_default(cls): if hasattr(select, "epoll"): from tornado.platform.epoll import EPollIOLoop return EPollIOLoop if hasattr(select, "kqueue"): # Python 2.6+ on BSD or Mac from tornado.platform.kqueue import KQueueIOLoop return KQueueIOLoop from tornado.platform.select import SelectIOLoop return SelectIOLoop 

原来这是个工厂函数，根据不同的操作系统返回不同的事件池（ linux 就是 epoll ， mac 返回 kqueue ，其他就返回普通的 select 。 kqueue 基本等同于 epoll ， 只是不同系统对其的不同实现）

现在线索转移到了 tornado.platform.epoll.EPollIOLoop 上，我们再来看看 EPollIOLoop:

tornado.platform.epoll.EPollIOLoop

import select from tornado.ioloop import PollIOLoop class EPollIOLoop(PollIOLoop): def initialize(self, **kwargs): super(EPollIOLoop, self).initialize(impl=select.epoll(), **kwargs) 

EPollIOLoop 完全继承自 PollIOLoop （注意这里是 PollIOLoop 不是 IOLoop）并只是在初始化时指定了 impl 是 epoll ，所以看起来我们用 IOLoop 初始化最后初始化的其实就是这个 PollIOLoop，所以接下来，我们真正需要理解和阅读的内容应该都在这里：

tornado.ioloop.PollIOLoop

class PollIOLoop(IOLoop): """Base class for IOLoops built around a select-like function. For concrete implementations, see `tornado.platform.epoll.EPollIOLoop` (Linux), `tornado.platform.kqueue.KQueueIOLoop` (BSD and Mac), or `tornado.platform.select.SelectIOLoop` (all platforms). """ def initialize(self, impl, time_func=None, **kwargs): super(PollIOLoop, self).initialize(**kwargs) self._impl = impl if hasattr(self._impl, 'fileno'): set_close_exec(self._impl.fileno()) self.time_func = time_func or time.time self._handlers = {} self._events = {} self._callbacks = [] self._callback_lock = threading.Lock() self._timeouts = [] self._cancellatiOns= 0 self._running = False self._stopped = False self._closing = False self._thread_ident = None self._blocking_signal_threshold = None self._timeout_counter = itertools.count() # Create a pipe that we send bogus data to when we want to wake # the I/O loop when it is idle self._waker = Waker() self.add_handler(self._waker.fileno(), lambda fd, events: self._waker.consume(), self.READ) def close(self, all_fds=False): with self._callback_lock: self._closing = True self.remove_handler(self._waker.fileno()) if all_fds: for fd, handler in self._handlers.values(): self.close_fd(fd) self._waker.close() self._impl.close() self._callbacks = None self._timeouts = None def add_handler(self, fd, handler, events): fd, obj = self.split_fd(fd) self._handlers[fd] = (obj, stack_context.wrap(handler)) self._impl.register(fd, events | self.ERROR) def update_handler(self, fd, events): fd, obj = self.split_fd(fd) self._impl.modify(fd, events | self.ERROR) def remove_handler(self, fd): fd, obj = self.split_fd(fd) self._handlers.pop(fd, None) self._events.pop(fd, None) try: self._impl.unregister(fd) except Exception: gen_log.debug("Error deleting fd from IOLoop", exc_info=True) def set_blocking_signal_threshold(self, seconds, action): if not hasattr(signal, "setitimer"): gen_log.error("set_blocking_signal_threshold requires a signal module " "with the setitimer method") return self._blocking_signal_threshold = seconds if seconds is not None: signal.signal(signal.SIGALRM, action if action is not None else signal.SIG_DFL) def start(self: ... try: while True: # Prevent IO event starvation by delaying new callbacks # to the next iteration of the event loop. with self._callback_lock: callbacks = self._callbacks self._callbacks = [] # Add any timeouts that have come due to the callback list. # Do not run anything until we have determined which ones # are ready, so timeouts that call add_timeout cannot # schedule anything in this iteration. due_timeouts = [] if self._timeouts: now = self.time() while self._timeouts: if self._timeouts[0].callback is None: # The timeout was cancelled. Note that the # cancellation check is repeated below for timeouts # that are cancelled by another timeout or callback. heapq.heappop(self._timeouts) self._cancellations -= 1 elif self._timeouts[0].deadline <= now: due_timeouts.append(heapq.heappop(self._timeouts)) else: break if (self._cancellations > 512 and self._cancellations > (len(self._timeouts) >> 1)): # Clean up the timeout queue when it gets large and it's # more than half cancellations. self._cancellatiOns= 0 self._timeouts = [x for x in self._timeouts if x.callback is not None] heapq.heapify(self._timeouts) for callback in callbacks: self._run_callback(callback) for timeout in due_timeouts: if timeout.callback is not None: self._run_callback(timeout.callback) # Closures may be holding on to a lot of memory, so allow # them to be freed before we go into our poll wait. callbacks = callback = due_timeouts = timeout = None if self._callbacks: # If any callbacks or timeouts called add_callback, # we don't want to wait in poll() before we run them. poll_timeout = 0.0 elif self._timeouts: # If there are any timeouts, schedule the first one. # Use self.time() instead of 'now' to account for time # spent running callbacks. poll_timeout = self._timeouts[0].deadline - self.time() poll_timeout = max(0, min(poll_timeout, _POLL_TIMEOUT)) else: # No timeouts and no callbacks, so use the default. poll_timeout = _POLL_TIMEOUT if not self._running: break if self._blocking_signal_threshold is not None: # clear alarm so it doesn't fire while poll is waiting for # events. signal.setitimer(signal.ITIMER_REAL, 0, 0) try: event_pairs = self._impl.poll(poll_timeout) except Exception as e: # Depending on python version and IOLoop implementation, # different exception types may be thrown and there are # two ways EINTR might be signaled: # * e.errno == errno.EINTR # * e.args is like (errno.EINTR, 'Interrupted system call') if errno_from_exception(e) == errno.EINTR: continue else: raise if self._blocking_signal_threshold is not None: signal.setitimer(signal.ITIMER_REAL, self._blocking_signal_threshold, 0) # Pop one fd at a time from the set of pending fds and run # its handler. Since that handler may perform actions on # other file descriptors, there may be reentrant calls to # this IOLoop that update self._events self._events.update(event_pairs) while self._events: fd, events = self._events.popitem() try: fd_obj, handler_func = self._handlers[fd] handler_func(fd_obj, events) except (OSError, IOError) as e: if errno_from_exception(e) == errno.EPIPE: # Happens when the client closes the connection pass else: self.handle_callback_exception(self._handlers.get(fd)) except Exception: self.handle_callback_exception(self._handlers.get(fd)) fd_obj = handler_func = None finally: # reset the stopped flag so another start/stop pair can be issued self._stopped = False if self._blocking_signal_threshold is not None: signal.setitimer(signal.ITIMER_REAL, 0, 0) IOLoop._current.instance = old_current if old_wakeup_fd is not None: signal.set_wakeup_fd(old_wakeup_fd) def stop(self): self._running = False self._stopped = True self._waker.wake() def time(self): return self.time_func() def call_at(self, deadline, callback, *args, **kwargs): timeout = _Timeout( deadline, functools.partial(stack_context.wrap(callback), *args, **kwargs), self) heapq.heappush(self._timeouts, timeout) return timeout def remove_timeout(self, timeout): # Removing from a heap is complicated, so just leave the defunct # timeout object in the queue (see discussion in # http://docs.python.org/library/heapq.html). # If this turns out to be a problem, we could add a garbage # collection pass whenever there are too many dead timeouts. timeout.callback = None self._cancellations += 1 def add_callback(self, callback, *args, **kwargs): with self._callback_lock: if self._closing: raise RuntimeError("IOLoop is closing") list_empty = not self._callbacks self._callbacks.append(functools.partial( stack_context.wrap(callback), *args, **kwargs)) if list_empty and thread.get_ident() != self._thread_ident: # If we're in the IOLoop's thread, we know it's not currently # polling. If we're not, and we added the first callback to an # empty list, we may need to wake it up (it may wake up on its # own, but an occasional extra wake is harmless). Waking # up a polling IOLoop is relatively expensive, so we try to # avoid it when we can. self._waker.wake() def add_callback_from_signal(self, callback, *args, **kwargs): with stack_context.NullContext(): if thread.get_ident() != self._thread_ident: # if the signal is handled on another thread, we can add # it normally (modulo the NullContext) self.add_callback(callback, *args, **kwargs) else: # If we're on the IOLoop's thread, we cannot use # the regular add_callback because it may deadlock on # _callback_lock. Blindly insert into self._callbacks. # This is safe because the GIL makes list.append atomic. # One subtlety is that if the signal interrupted the # _callback_lock block in IOLoop.start, we may modify # either the old or new version of self._callbacks, # but either way will work. self._callbacks.append(functools.partial( stack_context.wrap(callback), *args, **kwargs)) 

果然， PollIOLoop 继承自 IOLoop 并实现了它的所有接口，现在我们终于可以进入真正的正题了:joy:

ioloop 分析

首先要看的是关于 epoll 操作的方法，还记得前文说过的 epoll 只需要四个 api 就能完全操作嘛？ 我们来看 PollIOLoop 的实现：

epoll 操作

def add_handler(self, fd, handler, events): fd, obj = self.split_fd(fd) self._handlers[fd] = (obj, stack_context.wrap(handler)) self._impl.register(fd, events | self.ERROR) def update_handler(self, fd, events): fd, obj = self.split_fd(fd) self._impl.modify(fd, events | self.ERROR) def remove_handler(self, fd): fd, obj = self.split_fd(fd) self._handlers.pop(fd, None) self._events.pop(fd, None) try: self._impl.unregister(fd) except Exception: gen_log.debug("Error deleting fd from IOLoop", exc_info=True) 

epoll_ctl：这个三个方法分别对应 epoll_ctl 中的 add 、 modify 、 del 参数。 所以这三个方法实现了 epoll 的 epoll_ctl 。

epoll_create：然后 epoll 的生成在前文 EPollIOLoop 的初始化中就已经完成了：super(EPollIOLoop, self).initialize(impl=select.epoll(), **kwargs)。 这个相当于 epoll_create 。

epoll_wait： epoll_wait 操作则在 start() 中：event_pairs = self._impl.poll(poll_timeout)

epoll_close：而 epoll 的 close 则在 PollIOLoop 中的 close 方法内调用： self._impl.close() 完成。

initialize

接下来看 PollIOLoop 的初始化方法中作了什么：

def initialize(self, impl, time_func=None, **kwargs): super(PollIOLoop, self).initialize(**kwargs) self._impl = impl # 指定 epoll if hasattr(self._impl, 'fileno'): set_close_exec(self._impl.fileno()) # fork 后关闭无用文件描述符 self.time_func = time_func or time.time # 指定获取当前时间的函数 self._handlers = {} # handler 的字典，储存被 epoll 监听的 handler ，与打开它的文件描述符 ( file descriptor 简称 fd ) 一一对应 self._events = {} # event 的字典，储存 epoll 返回的活跃的 fd event pairs self._callbacks = [] # 储存各个 fd 回调函数的列表 self._callback_lock = threading.Lock() # 指定进程锁 self._timeouts = [] # 将是一个最小堆结构，按照超时时间从小到大排列的 fd 的任务堆（ 通常这个任务都会包含一个 callback ） self._cancellatiOns= 0 # 关于 timeout 的计数器 self._running = False # ioloop 是否在运行 self._stopped = False # ioloop 是否停止 self._closing = False # ioloop 是否关闭 self._thread_ident = None # 当前线程堆标识符 （ thread identify ） self._blocking_signal_threshold = None # 系统信号， 主要用来在 epoll_wait 时判断是否会有 signal alarm 打断 epoll self._timeout_counter = itertools.count() # 超时计数器 ( 暂时不是很明白具体作用，好像和前面的 _cancellations 有关系？ 请大神讲讲) self._waker = Waker() # 一个 waker 类，主要是对于管道 pipe 的操作，因为 ioloop 属于底层的数据操作，这里 epoll 监听的是 pipe self.add_handler(self._waker.fileno(), lambda fd, events: self._waker.consume(), self.READ) # 将管道加入 epoll 监听，对于 web server 初始化时只需要关心 READ 事件 

除了注释中的解释，还有几点补充：

1. close_exec 的作用： 子进程在 fork 出来的时候，使用了写时复制（ COW ， Copy-On-Write ）方式获得父进程的数据空间、 堆和栈副本，这其中也包括文件描述符。刚刚 fork 成功时，父子进程中相同的文件描述符指向系统文件表中的同一项，接着，一般我们会调用 exec 执行另一个程序，此时会用全新的程序替换子进程的正文，数据，堆和栈等。此时保存文件描述符的变量当然也不存在了，我们就无法关闭无用的文件描述符了。所以通常我们会 fork 子进程后在子进程中直接执行 close 关掉无用的文件描述符，然后再执行 exec 。 所以 close_exec 执行的其实就是 关闭 ＋ 执行的作用。 详情可以查看： 关于 linux 进程间的 close-on-exec 机制

2. Waker()： Waker 封装了对于管道 pipe 的操作：

   def set_close_exec(fd): flags = fcntl.fcntl(fd, fcntl.F_GETFD) fcntl.fcntl(fd, fcntl.F_SETFD, flags | fcntl.FD_CLOEXEC) def _set_nonblocking(fd): flags = fcntl.fcntl(fd, fcntl.F_GETFL) fcntl.fcntl(fd, fcntl.F_SETFL, flags | os.O_NONBLOCK) class Waker(interface.Waker): def __init__(self): r, w = os.pipe() _set_nonblocking(r) _set_nonblocking(w) set_close_exec(r) set_close_exec(w) self.reader = os.fdopen(r, "rb", 0) self.writer = os.fdopen(w, "wb", 0) def fileno(self): return self.reader.fileno() def write_fileno(self): return self.writer.fileno() def wake(self): try: self.writer.write(b"x") except IOError: pass def consume(self): try: while True: result = self.reader.read() if not result: break except IOError: pass def close(self): self.reader.close() self.writer.close() ``` 

可以看到 waker 把 pipe 分为读、 写两个管道并都设置了非阻塞和 close_exec。 注意wake(self)方法中：self.writer.write(b"x") 直接向管道中写入随意字符从而释放管道。

原文地址

作者：rapospectre