Level Triggered and Edge Triggered

The epoll event distribution interface is able to behave both as Edge Triggered ( ET ) and Level Triggered ( LT ). The difference between ET and LT event distribution mechanism can be described as follows. Suppose that this scenario happens :

If the RFD file descriptor has been added to the epoll interface using the EPOLLET flag, the call to epoll_wait(2) done in step 5 will probably hang because of the available data still present in the file input buffers and the remote peer might be expecting a response based on the data it already sent. The reason for this is that Edge Triggered event distribution delivers events only when events happens on the monitored file. So, in step 5 the caller might end up waiting for some data that is already present inside the input buffer. In the above example, an event on RFD will be generated because of the write done in 2 and the event is consumed in 3. Since the read operation done in 4 does not consume the whole buffer data, the call to epoll_wait(2) done in step 5 might lock indefinitely. The epoll interface, when used with the EPOLLET flag ( Edge Triggered ) should use non-blocking file descriptors to avoid having a blocking read or write starve the task that is handling multiple file descriptors. The suggested way to use epoll as an Edge Triggered (EPOLLET) interface is below, and possible pitfalls to avoid follow.

On the contrary, when used as a Level Triggered interface, epoll is by all means a faster poll(2), and can be used wherever the latter is used since it shares the same semantics. Since even with the Edge Triggered epoll multiple events can be generated up on receipt 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 receipt of an event with epoll_wait(2). When the EPOLLONESHOT flag is specified, it is caller responsibility to rearm the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.

Example for Suggested Usage

While the usage of epoll when employed like a Level Triggered interface does have the same semantics of poll(2), an Edge Triggered usage requires more clarification to avoid stalls 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 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.

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=%d\n",
                        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.

Questions and Answers

Possible Pitfalls and Ways to Avoid Them

The solution is to maintain a ready list and mark the file descriptor as ready in its associated data structure, thereby allowing the application to remember which files need to be processed but still round robin amongst all the ready files. This also supports ignoring subsequent events you receive for fd's that are already ready.

One solution for this is to call, during the processing of event 47, epoll_ctl(EPOLL_CTL_DEL) to delete fd 13 and close(2), then mark its associated data structure as removed and link it to a cleanup list. If you find another event for fd 13 in your batch processing, you will discover the fd had been previously removed and there will be no confusion.