I made the mistake of trying to upgrade to 4.1.x from the awesome, perfect 4.0.20 release, and after a while the server stopped responding to request, at 0% CPU usage.
strace -tt -T -p 31579:
10:51:58.847925 epoll_wait(6, {}, 10, 100) = 0 <0.096000>
10:51:58.943970...