摘要：如果一個事件被設置了，那么這個事件就是持續化的，意思就是這個事件會保持掛起狀態，即使回調函數被執行。任何時候事件的回調函數觸發都會重置持續化事件中的超時狀態。

從網上找的內容，可以看下github中的一本book，點擊下面的連接就ok了。
github-libevent_book引用其中的話

About Event Persistence
By default, whenever a pending event becomes active (because its fd is ready to read or write, or because its timeout expires), it becomes non-pending right before its callback is executed. Thus, if you want to make the event pending again, you can call event_add() on it again from inside the callback function.
If the EV_PERSIST flag is set on an event, however, the event is persistent. This means that event remains pending even when its callback is activated. If you want to make it non-pending from within its callback, you can call event_del() on it.
The timeout on a persistent event resets whenever the event’s callback runs. Thus, if you have an event with flags EV_READ|EV_PERSIST and a timeout of five seconds, the event will become active:

Whenever the socket is ready for reading.

Whenever five seconds have passed since the event last became active.

可能有不對的地方，歡迎指正

關于事件的持續化
默認情況下，任何時候一個掛起的事件被激活（因為他的fd準備好了讀或者寫，或者因為他的超時過期了），它會在回調函數執行之前變為非掛起。如果你想讓事件再次掛起，你需要在回調函數內部調用event_add()。
如果一個事件被設置了EV_PERSIST，那么這個事件就是持續化的，意思就是這個事件會保持掛起狀態，即使回調函數被執行。如果你想讓它變為非掛起狀態，可以在回調函數中調用event_del()。

任何時候事件的回調函數觸發都會重置持續化事件中的超時狀態。因此，如果的事件有EV_READ/EV_PERSIST并且設置了5秒超時，那么有兩種情況會觸發這個事件：

當socket可以進行讀取的時候

當5s超時到期的時候