epoll

Level-triggered and edge-triggered

The epoll event distribution interface is able to behave both as edge-triggered (ET) and as level-triggered (LT). The difference between the two mechanisms can be described as follows. Suppose that this scenario happens:

1.

The file descriptor that represents the read side of a pipe (rfd) is registered on the epoll instance.

2.

A pipe writer writes 2 kB of data on the write side of the pipe.

3.

A call to epoll_wait(2) is done that will return rfd as a ready file descriptor.

4.

The pipe reader reads 1 kB of data from rfd.

5.

A call to epoll_wait(2) is done.

If the rfd file descriptor has been added to the epoll interface using the EPOLLET (edge-triggered) flag, the call to epoll_wait(2) done in step 5 will probably hang despite the available data still present in the file input buffer; meanwhile the remote peer might be expecting a response based on the data it already sent. The reason for this is that edge-triggered mode delivers events only when changes occur on the monitored file descriptor. 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 block indefinitely.

An application that employs the EPOLLET flag should use nonblocking file descriptors to avoid having a blocking read or write starve a task that is handling multiple file descriptors. The suggested way to use epoll as an edge-triggered (EPOLLET) interface is as follows:

By contrast, when used as a level-triggered interface (the default, when EPOLLET is not specified), epoll is simply a faster poll(2), and can be used wherever the latter is used since it shares the same semantics.

Since even with edge-triggered epoll, multiple events can be generated upon 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 the caller's responsibility to rearm the file descriptor using epoll_ctl(2) with EPOLL_CTL_MOD.

Interaction with autosleep

If the system is in autosleep mode via /sys/power/autosleep and an event happens which wakes the device from sleep, the device driver will keep the device awake only until that event is queued. To keep the device awake until the event has been processed, it is necessary to use the epoll_ctl(2) EPOLLWAKEUP flag.

When the EPOLLWAKEUP flag is set in the events field for a struct epoll_event, the system will be kept awake from the moment the event is queued, through the epoll_wait(2) call which returns the event until the subsequent epoll_wait(2) call. If the event should keep the system awake beyond that time, then a separate wake_lock should be taken before the second epoll_wait(2) call.

/proc interfaces

The following interfaces can be used to limit the amount of kernel memory consumed by epoll:

/proc/sys/fs/epoll/max_user_watches (since Linux 2.6.28)

This specifies a limit on the total number of file descriptors that a user can register across all epoll instances on the system. The limit is per real user ID. Each registered file descriptor costs roughly 90 bytes on a 32-bit kernel, and roughly 160 bytes on a 64-bit kernel. Currently, the default value for max_user_watches is 1/25 (4%) of the available low memory, divided by the registration cost in bytes.

Example for suggested usage

While the usage of epoll when employed as a level-triggered interface does have the same semantics as poll(2), the edge-triggered usage requires more clarification to avoid stalls in the application event loop. In this example, listener is a nonblocking 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.

#define MAX_EVENTS 10
struct epoll_event ev, events[MAX_EVENTS];
int listen_sock, conn_sock, nfds, epollfd;
/* Code to set up listening socket, 'listen_sock',
   (socket(), bind(), listen()) omitted */
epollfd = epoll_create1(0);
if (epollfd == -1) {
    perror("epoll_create1");
    exit(EXIT_FAILURE);
}
ev.events = EPOLLIN;
ev.data.fd = listen_sock;
if (epoll_ctl(epollfd, EPOLL_CTL_ADD, listen_sock, &ev) == -1) {
    perror("epoll_ctl: listen_sock");
    exit(EXIT_FAILURE);
}
for (;;) {
    nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1);
    if (nfds == -1) {
        perror("epoll_wait");
        exit(EXIT_FAILURE);
    }
    for (n = 0; n < nfds; ++n) {
        if (events[n].data.fd == listen_sock) {
            conn_sock = accept(listen_sock,
                               (struct sockaddr *) &addr, &addrlen);
            if (conn_sock == -1) {
                perror("accept");
                exit(EXIT_FAILURE);
            }
            setnonblocking(conn_sock);
            ev.events = EPOLLIN | EPOLLET;
            ev.data.fd = conn_sock;
            if (epoll_ctl(epollfd, EPOLL_CTL_ADD, conn_sock,
                        &ev) == -1) {
                perror("epoll_ctl: conn_sock");
                exit(EXIT_FAILURE);
            }
        } 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

Q0

What is the key used to distinguish the file descriptors registered in an epoll set?

A0

The key is the combination of the file descriptor number and the open file description (also known as an "open file handle", the kernel's internal representation of an open file).

Q1

What happens if you register the same file descriptor on an epoll instance twice?

A1

You will probably get EEXIST. However, it is possible to add a duplicate (dup(2), dup2(2), fcntl(2) F_DUPFD) file descriptor to the same epoll instance. This can be a useful technique for filtering events, if the duplicate file descriptors are registered with different events masks.

Q2

Can two epoll instances wait for the same file descriptor? If so, are events reported to both epoll file descriptors?

A2

Yes, and events would be reported to both. However, careful programming may be needed to do this correctly.

Q3

Is the epoll file descriptor itself poll/epoll/selectable?

A3

Yes. If an epoll file descriptor has events waiting, then it will indicate as being readable.

Q4

What happens if one attempts to put an epoll file descriptor into its own file descriptor set?

A4

The epoll_ctl(2) call fails (EINVAL). However, you can add an epoll file descriptor inside another epoll file descriptor set.

Q5

Can I send an epoll file descriptor over a UNIX domain socket to another process?

A5

Yes, but it does not make sense to do this, since the receiving process would not have copies of the file descriptors in the epoll set.

Q6

Will closing a file descriptor cause it to be removed from all epoll sets automatically?

A6

Yes, but be aware of the following point. A file descriptor is a reference to an open file description (see open(2)). Whenever a file descriptor is duplicated via dup(2), dup2(2), fcntl(2) F_DUPFD, or fork(2), a new file descriptor referring to the same open file description is created. An open file description continues to exist until all file descriptors referring to it have been closed. A file descriptor is removed from an epoll set only after all the file descriptors referring to the underlying open file description have been closed (or before if the file descriptor is explicitly removed using epoll_ctl(2) EPOLL_CTL_DEL). This means that even after a file descriptor that is part of an epoll set has been closed, events may be reported for that file descriptor if other file descriptors referring to the same underlying file description remain open.

Q7

If more than one event occurs between epoll_wait(2) calls, are they combined or reported separately?

A7

They will be combined.

Q8

Does an operation on a file descriptor affect the already collected but not yet reported events?

A8

You can do two operations on an existing file descriptor. Remove would be meaningless for this case. Modify will reread available I/O.

Q9

Do I need to continuously read/write a file descriptor until EAGAIN when using the EPOLLET flag (edge-triggered behavior) ?

A9

Receiving an event from epoll_wait(2) should suggest to you that such file descriptor is ready for the requested I/O operation. You must consider it ready until the next (nonblocking) read/write yields EAGAIN. When and how you will use the file descriptor is entirely up to you.

For packet/token-oriented files (e.g., datagram socket, terminal in canonical mode), the only way to detect the end of the read/write I/O space is to continue to read/write until EAGAIN.

For stream-oriented files (e.g., pipe, FIFO, stream socket), the condition that the read/write I/O space is exhausted can also be detected by checking the amount of data read from / written to the target file descriptor. For example, if you call read(2) by asking to read a certain amount of data and read(2) returns a lower number of bytes, you can be sure of having exhausted the read I/O space for the file descriptor. The same is true when writing using write(2). (Avoid this latter technique if you cannot guarantee that the monitored file descriptor always refers to a stream-oriented file.)

Possible pitfalls and ways to avoid them

o Starvation (edge-triggered)

If there is a large amount of I/O space, it is possible that by trying to drain it the other files will not get processed causing starvation. (This problem is not specific to epoll.)

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 file descriptors that are already ready.

o If using an event cache...

If you use an event cache or store all the file descriptors returned from epoll_wait(2), then make sure to provide a way to mark its closure dynamically (i.e., caused by a previous event's processing). Suppose you receive 100 events from epoll_wait(2), and in event #47 a condition causes event #13 to be closed. If you remove the structure and close(2) the file descriptor for event #13, then your event cache might still say there are events waiting for that file descriptor causing confusion.

One solution for this is to call, during the processing of event 47, epoll_ctl(EPOLL_CTL_DEL) to delete file descriptor 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 file descriptor 13 in your batch processing, you will discover the file descriptor had been previously removed and there will be no confusion.