例子：
import time
import gevent
from gevent.threadpool import ThreadPool
pool = ThreadPool(3)
start = time.time()
for _ in range(4):
pool.spawn(time.sleep, 1)
gevent.wait()
delay = time.time() - start
print('Running "time.sleep(1)" 4 times with 3 threads. Should take about 2 seconds: %.3fs' % delay)
运行结果：
3个线程执行4个睡1s的操作：
（1）第一次，3个睡1s。使用了3个线程，满了，耗时1s
（2）第二次，1个睡1s。使用1个线程，耗时1s
（3）中间切换需要0.00x秒。
（4）理论总共耗时(2+0.00x)秒。
Running "time.sleep(1)" 4 times with 3 threads. Should take about 2 seconds: 2.004s
Process finished with exit code 0
