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epoll(7) — Linux manual page

epoll - I/O event notification facility

NOTE:

显然epoll的本质是**event notifier**

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.

epoll instance

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:

1、The interest list (sometimes also called the epoll set): the set of file descriptors that the process has registered an interest in monitoring.

2、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 that is dynamically populated by the kernel as a result of I/O activity on those file descriptors.

System calls

The following system calls are provided to create and manage an epoll instance:

1、epoll_create(2) creates an epoll instance and returns a file descriptor referring to that instance. (The more recent epoll_create1(2) extends the functionality of epoll_create(2).)

2、Interest in particular file descriptors is then registered via epoll_ctl(2). The set of file descriptors currently registered on an epoll instance is sometimes called an ***epoll* set**.

3、epoll_wait(2) waits for I/O events, blocking the calling thread if no events are currently available.

NOTE:

如果有event已经available了，则立即返回这个event对应的fd。

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.

NOTE:

写入了 2 kB，但是仅仅读出了 1 kB，显然，没有将所有写入的data读出。

5、A call to epoll_wait(2) is done.

EPOLLET (edge-triggered)

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(挂起，其实就是阻塞) 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.

如果已使用EPOLLET（边缘触发）标志将rfd文件描述符添加到epoll接口，则尽管文件输入缓冲区中仍存在可用数据，但在步骤5中完成的对epoll_wait（2）的调用可能会挂起; 同时，远程对等体可能期望基于其已发送的数据进行响应。 原因是边缘触发模式仅在受监视文件描述符发生更改时才传递事件。 因此，在步骤5中，调用者可能最终等待输入缓冲区内已存在的某些数据。 在上面的示例中，将生成rfd上的事件，因为写入在2中完成并且事件在3中消耗。由于在4中完成的读取操作不消耗整个缓冲区数据，因此对epoll_wait（2）的调用已完成 在步骤5中可能会无限期地阻止。

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:

i with nonblocking file descriptors; and

ii by waiting for an event only after read(2) or write(2) return EAGAIN.

NOTE : 要理解上面这段话的含义，就需要搞清楚blocking IO和nonblocking IO，以及它们和epoll之间的关系，参考下面的两篇文章：

1、Why having to use non-blocking fd in a edge triggered epoll function?

2、Blocking I/O, Nonblocking I/O, And Epoll

level-triggered

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.

EPOLLONESHOT

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.

Avoiding "thundering herd" wake-ups

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 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.

NOTE:

一、"thundering herd" 是 "惊群效应"，参见:

Parallel-computing\docs\Concurrent-computing\Classic-problem\Thundering-herd-problem

二、需要注意的是: 在比较旧版本中，"thundering herd" wake-ups 依然是存在的

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(抛锚) 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

Possible pitfalls and ways to avoid them o Starvation (edge-triggered)