The epoll API performs a similar task to poll(2): monitoring
multiple file descriptors to see if I/O is possible on any of
them. The epoll API can be used either as an edge-triggered or a
level-triggered interface and scales well to large numbers of
watched file descriptors.
The central concept of the epoll API is the epoll instance, an
in-kernel data structure which, from a user-space perspective,
can be considered as a container for two lists:
• The interest list (sometimes also called the epoll set): the
set of file descriptors that the process has registered an
interest in monitoring.
• The ready list: the set of file descriptors that are "ready"
for I/O. The ready list is a subset of (or, more precisely, a
set of references to) the file descriptors in the interest
list. The ready list is dynamically populated by the kernel as
a result of I/O activity on those file descriptors.
The following system calls are provided to create and manage an
epoll instance:
• epoll_create(2) creates a new epoll instance and returns a file
descriptor referring to that instance. (The more recent
epoll_create1(2) extends the functionality of epoll_create(2).)
• Interest in particular file descriptors is then registered via
epoll_ctl(2), which adds items to the interest list of the
epoll instance.
• epoll_wait(2) waits for I/O events, blocking the calling thread
if no events are currently available. (This system call can be
thought of as fetching items from the ready list of the epoll
instance.)
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:
a) with nonblocking file descriptors; and
b) by waiting for an event only after read(2) or write(2) return
EAGAIN.
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.
If multiple threads (or processes, if child processes have
inherited the epoll file descriptor across fork(2)) are blocked
in epoll_wait(2) waiting on the same epoll file descriptor and a
file descriptor in the interest list that is marked for edge-
triggered (EPOLLET) notification becomes ready, just one of the
threads (or processes) is awoken from epoll_wait(2). This
provides a useful optimization for avoiding "thundering herd"
wake-ups in some scenarios.
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
0. What is the key used to distinguish the file descriptors
registered in an interest list?
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).
1. What happens if you register the same file descriptor on an
epoll instance twice?
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.
2. Can two epoll instances wait for the same file descriptor?
If so, are events reported to both epoll file descriptors?
Yes, and events would be reported to both. However, careful
programming may be needed to do this correctly.
3. Is the epoll file descriptor itself poll/epoll/selectable?
Yes. If an epoll file descriptor has events waiting, then it
will indicate as being readable.
4. What happens if one attempts to put an epoll file descriptor
into its own file descriptor set?
The epoll_ctl(2) call fails (EINVAL). However, you can add
an epoll file descriptor inside another epoll file descriptor
set.
5. Can I send an epoll file descriptor over a UNIX domain socket
to another process?
Yes, but it does not make sense to do this, since the
receiving process would not have copies of the file
descriptors in the interest list.
6. Will closing a file descriptor cause it to be removed from
all epoll interest lists?
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 interest list only after
all the file descriptors referring to the underlying open
file description have been closed. This means that even
after a file descriptor that is part of an interest list has
been closed, events may be reported for that file descriptor
if other file descriptors referring to the same underlying
file description remain open. To prevent this happening, the
file descriptor must be explicitly removed from the interest
list (using epoll_ctl(2) EPOLL_CTL_DEL) before it is
duplicated. Alternatively, the application must ensure that
all file descriptors are closed (which may be difficult if
file descriptors were duplicated behind the scenes by library
functions that used dup(2) or fork(2)).
7. If more than one event occurs between epoll_wait(2) calls,
are they combined or reported separately?
They will be combined.
8. Does an operation on a file descriptor affect the already
collected but not yet reported events?
You can do two operations on an existing file descriptor.
Remove would be meaningless for this case. Modify will
reread available I/O.
9. Do I need to continuously read/write a file descriptor until
EAGAIN when using the EPOLLET flag (edge-triggered behavior)?
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.
