Why the amount of greenlets will impact the elapsed time of the responses

问题

I'm using Python coroutine library gevent and monkey patch to increase the concurrency of http requests. But I noticed the elapsed time of the responses increased while the concurrency increased. Below the sample code:

import gevent
from gevent import monkey
import requests

monkey.patch_all(thread=False)


def action():
    resp = requests.get("https://www.google.com")
    if resp.status_code == 200:
        print resp.elapsed.total_seconds()


jobs = []
for i in range(100):
    jobs.append(gevent.spawn(action))

gevent.joinall(jobs)

When 10 greenlets were spawned, the elapsed time was around 0.9 seconds, but when the greenlets number was increased to 100, the elapsed time was around 1.6 ~ 2.0 seconds. Why this happened?

回答1:

greenlets are still single threaded, meaning they can only do one thing at a time. For any process that is cpu intensive this will incur a delay. Yes it is asynchronous, but it is not multiprocessed, so if something uses 1 second of CPU, you have delayed the results of any subsequent greenlet by 1 second.

So as your queue grows, the delay even if just ms, becomes noticeable.

来源：https://stackoverflow.com/questions/50209009/why-the-amount-of-greenlets-will-impact-the-elapsed-time-of-the-responses