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.)
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:
- (1)
- with nonblocking file descriptors; and
- (2)
- 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.
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.
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.
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.
- •
- 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).
- •
- 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.
- •
- 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.
- •
- 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.
- •
- 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.
- •
- 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.
- •
- 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)).
- •
- If more than one event occurs between epoll_wait(2) calls, are they
combined or reported separately?
- They will be combined.
- •
- 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.
- •
- 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.)
- •
- 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.
- •
- 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.