Eventloop 是什么？是为了解决什么问题而设计的？

Question

1.请问下，Eventloop 是什么？是为了解决什么问题而设计的？

2.下面是Python 版本的Eventloop 实现示例，这里的Eventloop和Javascript中的Eventloop要解决的问题是一样的吗，有什么区别吗？：

class KqueueLoop(object):

    MAX_EVENTS = 1024

    def __init__(self):
        self._kqueue = select.kqueue()
        self._fds = {}

    def _control(self, fd, mode, flags):
        events = []
        if mode & POLL_IN:
            events.append(select.kevent(fd, select.KQ_FILTER_READ, flags))
        if mode & POLL_OUT:
            events.append(select.kevent(fd, select.KQ_FILTER_WRITE, flags))
        for e in events:
            self._kqueue.control([e], 0)

    def poll(self, timeout):
        if timeout < 0:
            timeout = None  # kqueue behaviour
        events = self._kqueue.control(None, KqueueLoop.MAX_EVENTS, timeout)
        results = defaultdict(lambda: POLL_NULL)
        for e in events:
            fd = e.ident
            if e.filter == select.KQ_FILTER_READ:
                results[fd] |= POLL_IN
            elif e.filter == select.KQ_FILTER_WRITE:
                results[fd] |= POLL_OUT
        return results.items()

    def register(self, fd, mode):
        self._fds[fd] = mode
        self._control(fd, mode, select.KQ_EV_ADD)

    def unregister(self, fd):
        self._control(fd, self._fds[fd], select.KQ_EV_DELETE)
        del self._fds[fd]

    def modify(self, fd, mode):
        self.unregister(fd)
        self.register(fd, mode)

    def close(self):
        self.kqueue.close()


class SelectLoop(object):

    def __init__(self):
        self._r_list = set()
        self._w_list = set()
        self._x_list = set()

    def poll(self, timeout):
        r, w, x = select.select(self._r_list, self._w_list, self._x_list,
                                timeout)
        results = defaultdict(lambda: POLL_NULL)
        for p in [(r, POLL_IN), (w, POLL_OUT), (x, POLL_ERR)]:
            for fd in p[0]:
                results[fd] |= p[1]
        return results.items()

    def register(self, fd, mode):
        if mode & POLL_IN:
            self._r_list.add(fd)
        if mode & POLL_OUT:
            self._w_list.add(fd)
        if mode & POLL_ERR:
            self._x_list.add(fd)

    def unregister(self, fd):
        if fd in self._r_list:
            self._r_list.remove(fd)
        if fd in self._w_list:
            self._w_list.remove(fd)
        if fd in self._x_list:
            self._x_list.remove(fd)

    def modify(self, fd, mode):
        self.unregister(fd)
        self.register(fd, mode)

    def close(self):
        pass


class EventLoop(object):
    def __init__(self):
        if hasattr(select, 'epoll'):
            self._impl = select.epoll()
            model = 'epoll'
        elif hasattr(select, 'kqueue'):
            self._impl = KqueueLoop()
            model = 'kqueue'
        elif hasattr(select, 'select'):
            self._impl = SelectLoop()
            model = 'select'
        else:
            raise Exception('can not find any available functions in select '
                            'package')
        self._fdmap = {}  # (f, handler)
        self._last_time = time.time()
        self._periodic_callbacks = []
        self._stopping = False
        logging.debug('using event model: %s', model)

    def poll(self, timeout=None):
        events = self._impl.poll(timeout)
        return [(self._fdmap[fd][0], fd, event) for fd, event in events]

    def add(self, f, mode, handler):
        fd = f.fileno()
        self._fdmap[fd] = (f, handler)
        self._impl.register(fd, mode)

    def remove(self, f):
        fd = f.fileno()
        del self._fdmap[fd]
        self._impl.unregister(fd)

    def add_periodic(self, callback):
        self._periodic_callbacks.append(callback)

    def remove_periodic(self, callback):
        self._periodic_callbacks.remove(callback)

    def modify(self, f, mode):
        fd = f.fileno()
        self._impl.modify(fd, mode)

    def stop(self):
        self._stopping = True

    def run(self):
        events = []
        while not self._stopping:
            asap = False
            try:
                events = self.poll(TIMEOUT_PRECISION)
            except (OSError, IOError) as e:
                if errno_from_exception(e) in (errno.EPIPE, errno.EINTR):
                    # EPIPE: Happens when the client closes the connection
                    # EINTR: Happens when received a signal
                    # handles them as soon as possible
                    asap = True
                    logging.debug('poll:%s', e)
                else:
                    logging.error('poll:%s', e)
                    import traceback
                    traceback.print_exc()
                    continue

            for sock, fd, event in events:
                handler = self._fdmap.get(fd, None)
                if handler is not None:
                    handler = handler[1]
                    try:
                        handler.handle_event(sock, fd, event)
                    except (OSError, IOError) as e:
                        shell.print_exception(e)
            now = time.time()
            if asap or now - self._last_time >= TIMEOUT_PRECISION:
                for callback in self._periodic_callbacks:
                    callback()
                self._last_time = now

    def __del__(self):
        self._impl.close()