EPOLL
Section: Linux Programmer's Manual (7)
Updated: 2021-03-22
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NAME
epoll - I/O event notification facility
SYNOPSIS
#include <sys/epoll.h>
DESCRIPTION
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.
VERSIONS
The
epoll
API was introduced in Linux kernel 2.5.44.
Support was added to glibc in version 2.3.2.
CONFORMING TO
The
epoll
API is Linux-specific.
Some other systems provide similar
mechanisms, for example, FreeBSD has
kqueue,
and Solaris has
/dev/poll.
NOTES
The set of file descriptors that is being monitored via
an epoll file descriptor can be viewed via the entry for
the epoll file descriptor in the process's
/proc/[pid]/fdinfo
directory.
See
proc(5)
for further details.
The
kcmp(2)
KCMP_EPOLL_TFD
operation can be used to test whether a file descriptor
is present in an epoll instance.
SEE ALSO
epoll_create(2),
epoll_create1(2),
epoll_ctl(2),
epoll_wait(2),
poll(2),
select(2)
COLOPHON
This page is part of release 5.11 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Level-triggered and edge-triggered
-
- Interaction with autosleep
-
- /proc interfaces
-
- Example for suggested usage
-
- Questions and answers
-
- Possible pitfalls and ways to avoid them
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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Time: 06:22:50 GMT, May 09, 2021