Why the python script process enter the sleep state and can't be woken up for execution?

This script is using grequests concurrent request url. And when it runs for 33 min, it goes to sleep permanently. And I don't no why. This script used to run well.

The main code is, :

urlList.append(grequests.get(url))
num = num + 1
if num = 50:
    resList = grequests.map(urlList, exception_handler=err_handler)
    num = 0
    urlList = []

The process status：

ps aux | grep build.py

work     27647  3.8  0.0 972736 29476 ?        Sl   Jan27  33:26 python build.py

strace -p 27647


Process 27647 attached
clock_gettime(CLOCK_MONOTONIC, {1543747, 295274224}) = 0
clock_gettime(CLOCK_MONOTONIC, {1543747, 295337559}) = 0
epoll_wait(11, {}, 64, 59743)           = 0
clock_gettime(CLOCK_MONOTONIC, {1543807, 97334455}) = 0
clock_gettime(CLOCK_MONOTONIC, {1543807, 97428028}) = 0
epoll_wait(11, {}, 64, 59743)           = 0
clock_gettime(CLOCK_MONOTONIC, {1543866, 900111931}) = 0
clock_gettime(CLOCK_MONOTONIC, {1543866, 900158914}) = 0

env:

CentOS release 6.3
Python 2.7.13
grequests                     0.6.0