One aspect of the epoll interface is that it is edge-triggered; it will only return a file descriptor as being available for I/O after a change has happened on that file descriptor. In other words, if you tell epoll to watch a particular socket for readability, and a certain amount of data is already available for that socket, epoll will block anyway. It will only flag that socket as being readable when new data shows up.
Edge-triggered interfaces have their own advantages and disadvantages. One of their disadvantages, as epoll author Davide Libenzi has discovered, would appear to be that many programmers do not understand edge-triggered interfaces.. Additionally, most existing applications are written for level-triggered interfaces (such as poll() and select()) instead. Rather than fight this tide, he has sent out which switches epoll over to level-triggered behaviour. A subsequent patch makes the behaviour configurable on a per-file-descriptor basis.
Fantastic, level triggered interfaces are nicer because they need less system calls. With edge-triggered you always need to call read/write until it EAGAINs otherwise you can miss data. That means at least 2 calls per edge, while level triggered generally means only 1 call per edge.
Also, edge-triggering causes locking headaches when dealing with mutlithreaded apps and with these patches it should be possible to quite simply alter existing code to use epoll.