how to wake up a thread that blocking at epoll_wait?

I have two threads: one maintains a thread-safe message queue (handle this queue at the beginning of every loop) and deals with tcp connections, the other one posts message to the former one. the problem is, while the former one was blocking at epoll_wait, it's not sure that how long until the posted message be handled. So I want to wakeup the blocking thread when posting message to it.
Any suggestion?