說明：本文來自翻譯epoll man文檔。
1、ET，LT這兩種事件分發機制的不同。我們假定一個環境：
1. The file descriptor that represents the read side of a pipe ( RFD ) is added inside the epoll device.
2. Pipe writer writes 2Kb of data on the write side of the pipe.
3. A call to epoll_wait(2) is done that will return RFD as ready file descriptor.
4. The pipe reader reads 1Kb of data from RFD.
5. A call to epoll_wait(2) is done.
Edge Triggered 工作模式：
如果我們在第1步將RFD添加到epoll描述符的時候使用了EPOLLET標志,那么在第5步調用epoll_wait(2)之后將有可能會掛起，因為剩余的數據還存在于文件的輸入緩沖區內，而且數據發出端還在等待一個針對已經發出數據的反饋信息。只有在監視的文件句柄上發生了某個事件的時候，ET工作模式才會匯報事件（Edge Triggered event distribution delivers events）。因此在第5步的時候，調用者可能會放棄等待仍在存在于文件輸入緩沖區內的剩余數據（the caller might end up waiting for some data that is already present inside the input buffer.）。在上面的例子中，會有一個事件產生在RFD句柄上，是因為在第2步執行了一個寫操作，然后，事件將會在第3步被銷毀（consumed）。因為第4步的讀取操作沒有讀空文件輸入緩沖區內的數據，因此我們在第5步調用 epoll_wait(2)完成后，might lock indefinitely。epoll工作在ET模式的時候，必須使用非阻塞套接口，以避免由于一個文件句柄的阻塞讀/阻塞寫操作把處理多個文件描述符的任務餓死。最好以下面的方式調用ET模式的epoll接口，在后面會介紹避免可能的缺陷。
i 基于非阻塞文件句柄
ii 只有當read(2)或者write(2)返回EAGAIN時才需要掛起、等待（意為此時，緩存區滿或無數據）。
Level Triggered 工作模式
相反的，以LT方式調用epoll接口的時候,它就相當于一個速度比較快的poll，and can be used wherever the latter is used since it shares the same semantics。因為即使使用ET模式的epoll，在收到多個chunk的數據的時候仍然會產生多個事件（Since even with the Edge Triggered epoll multiple events can be generated up on receival of multiple chunks of data）。the caller has the option to specify the EPOLLONESHOT flag, to tell epoll to disable the associated file descriptor after the receival of an event with epoll_wait(2). When the EPOLLONESHOT flag is specified, it is caller responsibility to rearm（重新設置） the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.
2、While the usage of epoll when employed（使用） like a Level Triggered interface does have the same semantics of poll(2), an Edge Triggered usage requires more clarification（澄清） to avoid stalls（拖延） in the application event loop.
In this example, listener is a non-blocking socket on which listen(2) has been called. The function do_use_fd() uses the new ready file descriptor until EAGAIN is returned by either read(2) or write(2). An event driven state machine（事件驅動狀態機） application should, after having received EAGAIN, record its current state so that at the next call to do_use_fd() it will continue to read(2) or write(2) from where it stopped before.
示例代碼
View Code
struct epoll_event ev, *events;
for(;;) 
{
nfds = epoll_wait(kdpfd, events, maxevents, -1);
for(n = 0; n < nfds; ++n) {
if(events[n].data.fd == listener) {
client = accept(listener, (struct sockaddr *) &local,
&addrlen);
if(client < 0){
perror("accept");
continue;
}
setnonblocking(client);
ev.events = EPOLLIN | EPOLLET;
ev.data.fd = client;
if (epoll_ctl(kdpfd, EPOLL_CTL_ADD, client, &ev) < 0) {
fprintf(stderr, "epoll set insertion error: fd=%d0,
client);
return -1;
}
}
else
do_use_fd(events[n].data.fd);
}
}
When used as an Edge triggered interface, for performance reasons, it is possible to add the file descriptor inside the epoll interface ( EPOLL_CTL_ADD ) once by specifying ( EPOLLIN|EPOLLOUT ). This allows you to avoid continuously switching between EPOLLIN and EPOLLOUT calling epoll_ctl(2) with EPOLL_CTL_MOD.
水平觸發（LT, Level Triggered），默認方式
支持阻塞/非阻塞socket。
內核通知某fd就緒，如果不對fd操作，內核會繼續通知
邊緣觸發（Edge-Triggered）
只支持非阻塞socket
內核通知某fd就緒，如果不對fd操作，內核不再繼續通知