Python Asyncio调度原理是什么

# 已经移除了一些不想关的代码
class Handle:
    def __init__(self, callback, args, loop, context=None):
        # 初始化上下文，确保执行的时候能找到Handle所在的上下文
        if context is None:
            context = contextvars.copy_context()
        self._context = context
        self._loop = loop
        self._callback = callback
        self._args = args
        self._cancelled = False

    def cancel(self):
        # 设置当前Handle为取消状态
        if not self._cancelled:
            self._cancelled = True
            self._callback = None
            self._args = None
    def cancelled(self):
        return self._cancelled
    def _run(self):
        # 用于执行真正的函数，且通过context.run方法来确保在自己的上下文内执行。
        try:
            # 保持在自己持有的上下文中执行对应的回调
            self._context.run(self._callback, *self._args)
        except (SystemExit, KeyboardInterrupt):
            raise
        except BaseException as exc:
            cb = fORMat_helpers._format_callback_source(
                self._callback, self._args)
            msg = f'Exception in callback {cb}'
            context = {
                'message': msg,
                'exception': exc,
                'handle': self,
            }
            self._loop.call_exception_handler(context)

class TimerHandle(Handle):
    def __init__(self, when, callback, args, loop, context=None):
        super().__init__(callback, args, loop, context)
        self._when = when
        self._scheduled = False
    def __hash__(self):
        return hash(self._when)
    def __lt__(self, other):
        if isinstance(other, TimerHandle):
            return self._when < other._when
        return NotImplemented
    def __le__(self, other):
        if isinstance(other, TimerHandle):
            return self._when < other._when or self.__eq__(other)
        return NotImplemented
    def __gt__(self, other):
        if isinstance(other, TimerHandle):
            return self._when > other._when
        return NotImplemented
    def __ge__(self, other):
        if isinstance(other, TimerHandle):
            return self._when > other._when or self.__eq__(other)
        return NotImplemented
    def __eq__(self, other):
        if isinstance(other, TimerHandle):
            return (self._when == other._when and
                    self._callback == other._callback and
                    self._args == other._args and
                    self._cancelled == other._cancelled)
        return NotImplemented
    def cancel(self):
        if not self._cancelled:
            # 用于通知事件循环当前Handle已经退出了
            self._loop._timer_handle_cancelled(self)
        super().cancel()
    def when(self):
        return self._when

class BaseEventLoop:
    ...
    def call_soon(self, callback, *args, context=None):
        # 检查是否事件循环是否关闭，如果是则直接抛出异常
        self._check_closed()
        handle = self._call_soon(callback, args, context)
        return handle

   def _call_soon(self, callback, args, context):
        # 把调用封装成一个handle，这样方便被事件循环调用
        handle = events.Handle(callback, args, self, context)
        # 添加一个handle到_ready，等待被调用
        self._ready.append(handle)
        return handle

class BaseEventLoop:
    ...
    def call_later(self, delay, callback, *args, context=None):
        if delay is None:
            raise TypeError('delay must not be None')
        timer = self.call_at(self.time() + delay, callback, *args, context=context)
        return timer

    def call_at(self, when, callback, *args, context=None):
        if when is None:
            raise TypeError("when cannot be None")
        self._check_closed()
        # 创建一个timer handle，然后添加到事件循环的_scheduled中，等待被调用
        timer = events.TimerHandle(when, callback, args, self, context)
        heapq.heappush(self._scheduled, timer)
        timer._scheduled = True
        return timer

class BaseEventLoop:
    def run_until_complete(self, future):
        ...
        new_task = not futures.isfuture(future)
        # 把coroutine转换成task，这样事件循环就可以调度了，事件循环的最小调度单位为task
        # 需要注意的是此时事件循环并没注册到全局变量中，所以需要显示的传进去，
        # 同时Task对象注册的时候，已经通过loop.call_soon把自己注册到事件循环中，等待调度
        future = tasks.ensure_future(future, loop=self)
        if new_task:
            # An exception is raised if the future didn't complete, so there
            # is no need to log the "destroy pending task" message
            future._log_destroy_pending = False
        # 当该task完成时，意味着当前事件循环失去了调度对象，无法继续调度，所以需要关闭当前事件循环，程序会由协程模式返回到线程模式
        future.add_done_callback(_run_until_complete_cb)
        try:
            # 事件循环开始运行
            self.run_forever()
        except:
            if new_task and future.done() and not future.cancelled():
                # The coroutine raised a BaseException. Consume the exception
                # to not log a warning, the caller doesn't have access to the
                # local task.
                future.exception()
            raise
        finally:
            future.remove_done_callback(_run_until_complete_cb)
        if not future.done():
            raise RuntimeError('Event loop stopped before Future completed.')

        return future.result()

    def run_forever(self):
        # 进行一些初始化工作
        self._check_closed()
        self._check_running()
        self._set_coroutine_origin_tracking(self._debug)
        self._thread_id = threading.get_ident()

        old_agen_hooks = sys.get_asyncgen_hooks()
        # 通过asyncgen钩子来自动关闭asyncgen函数，这样可以提醒用户生成器还未关闭
        sys.set_asyncgen_hooks(firstiter=self._asyncgen_firstiter_hook,
                               finalizer=self._asyncgen_finalizer_hook)
        try:
            # 设置当前在运行的事件循环到全局变量中，这样就可以在任一阶段获取到当前的事件循环了
            events._set_running_loop(self)
            while True:
                # 正真执行任务的逻辑
                self._run_once()
                if self._stopping:
                    break
        finally:
            # 关闭循环, 并且清理一些资源
            self._stopping = False
            self._thread_id = None
            events._set_running_loop(None)
            self._set_coroutine_origin_tracking(False)
            sys.set_asyncgen_hooks(*old_agen_hooks)

while True:
    # 正真执行任务的逻辑
    self._run_once()
    if self._stopping:
        break

class BaseEventLoop:
    ...
    def _run_once(self):
        # self._scheduled是一个列表，它只存放TimerHandle
        sched_count = len(self._scheduled)
        ###############################
        # 第一阶段，整理self._scheduled #
        ###############################
        if (sched_count > _MIN_SCHEDULED_TIMER_HANDLES and
            self._timer_cancelled_count / sched_count > _MIN_CANCELLED_TIMER_HANDLES_FRACTION):
            # 当待调度的任务数量超过100且待取消的任务占总任务的50%时，才进入这个逻辑
            # 把需要取消的任务移除
            new_scheduled = []
            for handle in self._scheduled:
                if handle._cancelled:
                    # 设置handle的_cancelled为True，并且把handle从_scheduled中移除
                    handle._scheduled = False
                else:
                    new_scheduled.append(handle)

            # 重新排列堆
            heapq.heapify(new_scheduled)
            self._scheduled = new_scheduled
            self._timer_cancelled_count = 0
        else:
            # 需要取消的handle不多，则只会走这个逻辑，这里会把堆顶的handle弹出，并标记为不可调度，但不会访问整个堆
            while self._scheduled and self._scheduled[0]._cancelled:
                self._timer_cancelled_count -= 1
                handle = heapq.heappop(self._scheduled)
                handle._scheduled = False

        #################################
        # 第二阶段，计算超时值以及等待事件IO #
        #################################
        timeout = None
        # 当有准备调度的handle或者是正在关闭时，不等待，方便尽快的调度
        if self._ready or self._stopping:
            timeout = 0
        elif self._scheduled:
            # Compute the desired timeout.
            # 如果堆有数据时，通过堆顶的handle计算最短的超时时间，但是最多不能超过MAXIMUM_SELECT_TIMEOUT，以免超过系统限制
            when = self._scheduled[0]._when
            timeout = min(max(0, when - self.time()), MAXIMUM_SELECT_TIMEOUT)

        # 事件循环等待事件，直到有事件或者超时
        event_list = self._selector.select(timeout)

        ##################################################
        # 第三阶段，把满足条件的TimeHandle放入到self._ready中 #
        ##################################################
        # 获取得到的事件的回调，然后装填到_ready
        self._process_events(event_list)

        # 把一些在self._scheduled且满足调度条件的handle放到_ready中，比如TimerHandle。
        # end_time为当前时间+一个时间单位，猜测是能多处理一些这段时间内产生的事件
        end_time = self.time() + self._clock_resolution
        while self._scheduled:
            handle = self._scheduled[0]
            if handle._when >= end_time:
                break
            handle = heapq.heappop(self._scheduled)
            handle._scheduled = False
            self._ready.append(handle)

        ################################################################################
        # 第四阶段，遍历所有准备调度的handle，并且通过handle的context来执行handle对应的callback #
        ################################################################################
        ntodo = len(self._ready)
        for i in range(ntodo):
            handle = self._ready.popleft()
            # 如果handle已经被取消，则不调用
            if handle._cancelled:
                continue
            if self._debug:
                try:
                    self._current_handle = handle
                    t0 = self.time()
                    handle._run()
                    dt = self.time() - t0
                    if dt >= self.slow_callback_duration:
                        # 执行太久的回调，记录下来，这些需要开发者自己优化
                        logger.warning('Executing %s took %.3f seconds',
                                       _format_handle(handle), dt)
                finally:
                    self._current_handle = None
            else:
                handle._run()
        handle = None  # Needed to break cycles when an exception occurs.

import asyncio
import threading
def task():
    print("task")
def run_loop_inside_thread(loop):
    loop.run_forever()
loop = asyncio.get_event_loop()
threading.Thread(target=run_loop_inside_thread, args=(loop,)).start()
loop.call_soon(task)

class BaseEventLoop:
    ...
    def call_soon_threadsafe(self, callback, *args, context=None):
        """Like call_soon(), but thread-safe."""
        self._check_closed()
        handle = self._call_soon(callback, args, context)
        self._write_to_self()
        return handle

class BaseSelectorEventLoop(base_events.BaseEventLoop):
    #######
    # 创建 #
    #######
    def __init__(self, selector=None):
        super().__init__()

        if selector is None:
            # 获取最优的selector
            selector = selectors.DefaultSelector()
        self._selector = selector
        # 创建pipe
        self._make_self_pipe()
        self._transports = weakref.WeakValueDictionary()
    def _make_self_pipe(self):
        # 创建Pipe对应的sock 
        self._ssock, self._csock = Socket.socketpair()
        # 设置sock为非阻塞
        self._ssock.setblocking(False)
        self._csock.setblocking(False)
        self._internal_fds += 1
        # 阻塞服务端sock读事件对应的回调
        self._add_reader(self._ssock.fileno(), self._read_from_self)
    def _add_reader(self, fd, callback, *args):
        # 检查事件循环是否关闭
        self._check_closed()
        # 封装回调为handle对象
        handle = events.Handle(callback, args, self, None)
        try:
            key = self._selector.get_key(fd)
        except KeyError:
            # 如果没有注册到系统的事件循环，则注册
            self._selector.reGISter(fd, selectors.EVENT_READ,
                                    (handle, None))
        else:
            # 如果已经注册过，则更新
            mask, (reader, writer) = key.events, key.data
            self._selector.modify(fd, mask | selectors.EVENT_READ,
                                  (handle, writer))
            if reader is not None:
                reader.cancel()
        return handle

    def _read_from_self(self):
        # 负责消费sock数据
        while True:
            try:
                data = self._ssock.recv(4096)
                if not data:
                    break
                self._process_self_data(data)
            except InterruptedError:
                continue
            except BlockingIOError:
                break
    #######
    # 删除 #
    #######
    def _close_self_pipe(self):
        # 注销Pipe对应的描述符 
        self._remove_reader(self._ssock.fileno())
        # 关闭sock
        self._ssock.close()
        self._ssock = None
        self._csock.close()
        self._csock = None
        self._internal_fds -= 1

    def _remove_reader(self, fd):
        # 如果事件循环已经关闭了，就不用操作了
        if self.is_closed():
            return False
        try:
            # 查询文件描述符是否在selector中
            key = self._selector.get_key(fd)
        except KeyError:
            # 不存在则返回
            return False
        else:
            # 存在则进入移除的工作
            mask, (reader, writer) = key.events, key.data
            # 通过事件掩码判断是否有其它事件
            mask &= ~selectors.EVENT_READ
            if not mask:
                # 移除已经注册到selector的文件描述符
                self._selector.unregister(fd)
            else:
                # 移除已经注册到selector的文件描述符，并注册新的事件
                self._selector.modify(fd, mask, (None, writer))

            # 如果reader不为空，则取消reader
            if reader is not None:
                reader.cancel()
                return True
            else:
                return False

    def _write_to_self(self):
        csock = self._csock
        if csock is None:
            return
        try:
            csock.send(b'\0')
        except OSError:
            if self._debug:
                logger.debug("Fail to write a null byte into the self-pipe socket", exc_info=True)

class BaseSelectorEventLoop:
    def _process_events(self, event_list):
        for key, mask in event_list:
            # 从回调事件中获取到对应的数据，key.data在注册时是一个元祖，所以这里要对元祖进行解包
            fileobj, (reader, writer) = key.fileobj, key.data
            if mask & selectors.EVENT_READ and reader is not None:
                # 得到reader handle，如果是被标记为取消，就移除对应的文件描述符
                if reader._cancelled:
                    self._remove_reader(fileobj)
                else:
                    # 如果没被标记为取消，则安排到self._ready中
                    self._add_callback(reader)
            if mask & selectors.EVENT_WRITE and writer is not None:
                # 对于写对象，也是同样的道理。
                if writer._cancelled:
                    self._remove_writer(fileobj)
                else:
                    self._add_callback(writer)

    def _add_callback(self, handle):
        # 把回调的handle添加到_ready中
        assert isinstance(handle, events.Handle), 'A Handle is required here'
        if handle._cancelled:
            return
        assert not isinstance(handle, events.TimerHandle)
        self._ready.append(handle)

    def _remove_reader(self, fd):
        # 如果事件循环已经关闭了，就不用操作了
        if self.is_closed():
            return False
        try:
            # 查询文件描述符是否在selector中
            key = self._selector.get_key(fd)
        except KeyError:
            # 不存在则返回
            return False
        else:
            # 存在则进入移除的工作
            mask, (reader, writer) = key.events, key.data
            mask &= ~selectors.EVENT_READ
            if not mask:
                # 移除已经注册到selector的文件描述符
                self._selector.unregister(fd)
            else:
                self._selector.modify(fd, mask, (None, writer))

            if reader is not None:
                reader.cancel()
                return True
            else:
                return False