Manpage of EPOLL

EPOLL

Section: Linux Programmer's Manual (7)
Updated: 2016-10-08
Index
 

NAME

epoll - I/O event notification facility  

SYNOPSIS

#include <sys/epoll.h> 

DESCRIPTION

The epollAPI performs a similar task to poll(2): monitoring multiple file descriptors to see if I/O is possible on any of them. The epollAPI can be used either as an edge-triggered or a level-triggered interface and scales well to large numbers of watched file descriptors. The following system calls are provided to create and manage an epollinstance:
*
epoll_create(2) creates an epollinstance 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). The set of file descriptors currently registered on an epollinstance is sometimes called an epollset.
*
epoll_wait(2) waits for I/O events, blocking the calling thread if no events are currently available.
 

Level-triggered and edge-triggered

The epollevent 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 epollinstance.
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 rfdas 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 rfdfile descriptor has been added to the epollinterface using the EPOLLET(edge-triggered) flag, the call to epoll_wait(2) done in step 5will 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 5the caller might end up waiting for some data that is already present inside the input buffer. In the above example, an event on rfdwill be generated because of the write done in 2and the event is consumed in 3. Since the read operation done in 4does not consume the whole buffer data, the call to epoll_wait(2) done in step 5might block indefinitely.

An application that employs the EPOLLETflag 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 epollas an edge-triggered (EPOLLET) interface is as follows:

i
with nonblocking file descriptors; and
ii
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 EPOLLETis not specified), epollis 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 EPOLLONESHOTflag, to tell epollto disable the associated file descriptor after the receipt of an event with epoll_wait(2). When the EPOLLONESHOTflag 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 autosleepmode via /sys/power/autosleepand 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) EPOLLWAKEUPflag.

When the EPOLLWAKEUPflag is set in the eventsfield 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_lockshould 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_watchesis 1/25 (4%) of the available low memory, divided by the registration cost in bytes.
 

Example for suggested usage

While the usage of epollwhen 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 EAGAINis 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, aqlisten_sockaq,
   (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 epollinterface (EPOLL_CTL_ADD) once by specifying (EPOLLIN|EPOLLOUT). This allows you to avoid continuously switching between EPOLLINand EPOLLOUTcalling epoll_ctl(2) with EPOLL_CTL_MOD.  

Questions and answers

Q0
What is the key used to distinguish the file descriptors registered in an epollset?
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 epollinstance 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 epollinstance. This can be a useful technique for filtering events, if the duplicate file descriptors are registered with different eventsmasks.
Q2
Can two epollinstances wait for the same file descriptor? If so, are events reported to both epollfile descriptors?
A2
Yes, and events would be reported to both. However, careful programming may be needed to do this correctly.
Q3
Is the epollfile descriptor itself poll/epoll/selectable?
A3
Yes. If an epollfile descriptor has events waiting, then it will indicate as being readable.
Q4
What happens if one attempts to put an epollfile descriptor into its own file descriptor set?
A4
The epoll_ctl(2) call will fail (EINVAL). However, you can add an epollfile descriptor inside another epollfile descriptor set.
Q5
Can I send an epollfile 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 epollset.
Q6
Will closing a file descriptor cause it to be removed from all epollsets 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 epollset 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 epollset 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 EAGAINwhen using the EPOLLETflag (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.  

VERSIONS

The epollAPI was introduced in Linux kernel 2.5.44. Support was added to glibc in version 2.3.2.  

CONFORMING TO

The epollAPI 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]/fdinfodirectory. See proc(5) for further details.  

SEE ALSO

epoll_create(2), epoll_create1(2), epoll_ctl(2), epoll_wait(2), poll(2), select(2)


 

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

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