Edge-triggered interfaces are too difficult?
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 a new patch which switches epoll over to level-triggered behavior. A subsequent patch makes the behavior configurable on a per-file-descriptor basis.
The end result is a more flexible epoll interface that can be more easily
used in existing applications. The patch has not been merged as of this
writing, but there does not seem to be any reason why it shouldn't be.
After all, epoll has not yet appeared in a stable kernel release; now is
the best time to be making improvements to the interface.
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