新手尝试用 aiohttp 写了个爬虫，但是目前因为 task 过多(超过 1000 个)，报错 Too many open files，请问如何解决呢？

完整代码

import time
import asyncio

import aiohttp
from bs4 import BeautifulSoup as bs

BASE_URL = "http://www.biqudu.com"
TITLE2URL = dict()
CONTENT = list()


async def fetch(url, callback=None, **kwarags):
    headers = {'User-Agent':'Mozilla/4.0 (compatible; MSIE 5.5; Windows NT)'}
    sem = asyncio.Semaphore(5)  
    with (await sem):
        async with aiohttp.ClientSession() as session: 
            async with session.get(url, headers=headers) as res:
                page = await res.text()
                if callback:
                    callback(page, **kwarags)
                else:
                    return page


def parse_url(page):
    soup = bs(page, "lxml")
    dd_a_doc = soup.select("dd > a")
    for a_doc in dd_a_doc:
        article_page_url = a_doc['href']
        article_title = a_doc.get_text()
        if article_page_url:
            TITLE2URL[article_title] = article_page_url


def parse_body(page, **kwarags):
    title = kwarags.get('title', '')
    print("{}".format(title))
    soup = bs(page, "lxml")
    content_doc = soup.find("div", id="content")
    content_text = content_doc.get_text().replace('readx();', '').replace('    ', "\r\n")
    content = "%s\n%s\n\n" % (title, content_text)
    CONTENT.append(content)


def main():
    t0 = time.time()
    loop = asyncio.get_event_loop()
    loop.run_until_complete(fetch(BASE_URL+"/43_43074/", callback=parse_url))
    tasks = [fetch(BASE_URL + page_url, callback=parse_body, title=title) for title, page_url in TITLE2URL.items()]
    loop.run_until_complete(asyncio.gather(*tasks[:500]))
    loop.close()
    elapsed = time.time() - t0
    print("cost {}".format(elapsed))


if __name__ == "__main__":
    main()

错误信息

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/connector.py", line 797, in _wrap_create_connection
    return (yield from self._loop.create_connection(*args, **kwargs))
  File "/usr/lib/python3.5/asyncio/base_events.py", line 695, in create_connection
    raise exceptions[0]
  File "/usr/lib/python3.5/asyncio/base_events.py", line 662, in create_connection
    sock = socket.socket(family=family, type=type, proto=proto)
  File "/usr/lib/python3.5/socket.py", line 134, in __init__
    _socket.socket.__init__(self, family, type, proto, fileno)
OSError: [Errno 24] Too many open files

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/xsy/Workspace/Self/aiotest/aiotest.py", line 58, in <module>
    main()
  File "/home/xsy/Workspace/Self/aiotest/aiotest.py", line 52, in main
    loop.run_until_complete(asyncio.gather(*tasks[:500]))
  File "/usr/lib/python3.5/asyncio/base_events.py", line 387, in run_until_complete
    return future.result()
  File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
    raise self._exception
  File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
    result = coro.send(None)
  File "/home/xsy/Workspace/Self/aiotest/aiotest.py", line 18, in fetch
    async with session.get(url, headers=headers) as res:
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/client.py", line 690, in __aenter__
    self._resp = yield from self._coro
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/client.py", line 267, in _request
    conn = yield from self._connector.connect(req)
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/connector.py", line 402, in connect
    proto = yield from self._create_connection(req)
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/connector.py", line 749, in _create_connection
    _, proto = yield from self._create_direct_connection(req)
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/connector.py", line 860, in _create_direct_connection
    raise last_exc
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/connector.py", line 832, in _create_direct_connection
    req=req, client_error=client_error)
  File "/usr/local/lib/python3.5/dist-packages/aiohttp/connector.py", line 804, in _wrap_create_connection
    raise client_error(req.connection_key, exc) from exc
aiohttp.client_exceptions.ClientConnectorError: Cannot connect to host www.biqudu.com:443 ssl:True [Too many open files]

目前我能想到的方法是

修改 linux 的最大文件打开数量限制
对 task 切片多次运行

但是这样解决都感觉太蠢了，请问有什么更好的方式吗？