nginx epoll详解

nginx epoll 事件模型

    nginx做为一个异步高效的事件驱动型web服务器，在linux平台中当系统支持epoll时nginx默认采用epoll来高效的处理事件。nginx中使用ngx_event_t结构来表示一个事件，先介绍下ngx_event_t结构体中成员的含义：

struct ngx_event_s {
    void            *data;  //与事件关联的对象，常指向事件所在的ngx_connection_t连接对象

    unsigned         write:1; //可写标识位，1表示对应的tcp连接是可写的

    unsigned         accept:1;// 1表示对应的连接是处于监听状态的连接，即可接收新的连接

    /* used to detect the stale events in kqueue, rtsig, and epoll */
    unsigned         instance:1; //可来区分事件是否已过期

    /*
     * the event was passed or would be passed to a kernel;
     * in aio mode - operation was posted.
     */
    unsigned         active:1;// 1表示事件活跃，即事件已添加到epoll中

    unsigned         disabled:1;//epoll中不使用该标识位

    /* the ready event; in aio mode 0 means that no operation can be posted */
    unsigned         ready:1; //事件已就绪(即可读或可写)

    unsigned         oneshot:1;//epoll不使用该标识位

    /* aio operation is complete */
    unsigned         complete:1;//aio中使用，表示 事件对应的aio异步操作已完成(io_getevents函数已成功返回)

    unsigned         eof:1;// 1表示当前处理的字符流已完成，如调用recv读取连接数据时返回0，此时置该标识位为1
    unsigned         error:1;// 1表示事件处理过程中发生错误

    unsigned         timedout:1;   //事件是否超时，1:表示超时。超时后事件对应的请求不需再被处理(对于http模块来说事件超时后直接关闭请求)
    unsigned         timer_set:1; //为1时表示这个事件在定时器中

    unsigned         delayed:1;// 1表示 需延迟处理该事件，常用于限速功能中

    unsigned         deferred_accept:1;//延迟接收接连，即当连接中收到对象发送的数据后才真正建立连接

    /* the pending eof reported by kqueue, epoll or in aio chain operation */
    unsigned         pending_eof:1;// 1表示TCP连接对向关闭读端，即epoll返回EPOLLRDHUP

#if !(NGX_THREADS)
    unsigned         posted_ready:1;//该标识位在1.5.5版本源码中只在ngx_epoll_process_events函数中有置位，其它地方并没有用到
#endif

#if (NGX_WIN32)
    /* setsockopt(SO_UPDATE_ACCEPT_CONTEXT) was successful */
    unsigned         accept_context_updated:1;
#endif

#if (NGX_HAVE_KQUEUE)
    unsigned         kq_vnode:1;

    /* the pending errno reported by kqueue */
    int              kq_errno;
#endif

    /*
     * kqueue only:
     *   accept:     number of sockets that wait to be accepted
     *   read:       bytes to read when event is ready
     *               or lowat when event is set with NGX_LOWAT_EVENT flag
     *   write:      available space in buffer when event is ready
     *               or lowat when event is set with NGX_LOWAT_EVENT flag
     *
     * iocp: TODO
     *
     * otherwise:
     *   accept:     1 if accept many, 0 otherwise
     */

#if (NGX_HAVE_KQUEUE) || (NGX_HAVE_IOCP)
    int              available;
#else
    unsigned         available:1;// 1表示每次调用accept时尽可能多的接收TCP连接，与multi_accept配置项对应
#endif

    ngx_event_handler_pt  handler; // 事件产生后的回调函数句柄


#if (NGX_HAVE_AIO)

#if (NGX_HAVE_IOCP)
    ngx_event_ovlp_t ovlp;
#else
    struct aiocb     aiocb;
#endif

#endif

    ngx_uint_t       index; //epoll中不使用

    ngx_log_t       *log; //ngx_log_t对象

    ngx_rbtree_node_t   timer;  

    unsigned         closed:1; // 1表示事件已关闭

    /* to test on worker exit */
    unsigned         channel:1;// 只在ngx_add_channel_event函数中有置位，其它地方没用到
    unsigned         resolver:1; // resolver功能中使用?

#if (NGX_THREADS)

    unsigned         locked:1;

    unsigned         posted_ready:1;
    unsigned         posted_timedout:1;
    unsigned         posted_eof:1;

#if (NGX_HAVE_KQUEUE)
    /* the pending errno reported by kqueue */
    int              posted_errno;
#endif

#if (NGX_HAVE_KQUEUE) || (NGX_HAVE_IOCP)
    int              posted_available;
#else
    unsigned         posted_available:1;
#endif

    ngx_atomic_t    *lock;
    ngx_atomic_t    *own_lock;

#endif

    /* the links of the posted queue */
    ngx_event_t     *next;
    ngx_event_t    **prev;


#if 0

    /* the threads support */

    /*
     * the event thread context, we store it here
     * if $(CC) does not understand __thread declaration
     * and pthread_getspecific() is too costly
     */

    void            *thr_ctx;

#if (NGX_EVENT_T_PADDING)

    /* event should not cross cache line in SMP */

    uint32_t         padding[NGX_EVENT_T_PADDING];
#endif
#endif
};


#if (NGX_HAVE_FILE_AIO)

struct ngx_event_aio_s {
    void                      *data;
    ngx_event_handler_pt       handler;
    ngx_file_t                *file;

    ngx_fd_t                   fd;

#if (NGX_HAVE_EVENTFD)
    int64_t                    res;
#if (NGX_TEST_BUILD_EPOLL)
    ngx_err_t                  err;
    size_t                     nbytes;
#endif
#else
    ngx_err_t                  err;
    size_t                     nbytes;
#endif

#if (NGX_HAVE_AIO_SENDFILE)
    off_t                      last_offset;
#endif

    ngx_aiocb_t                aiocb;
    ngx_event_t                event;
};

#endif

    nginx中使用ngx_epoll_module模块来封装epoll机制处理事件，ngx_epoll_module模块只对两个配置项感兴趣，其ngx_command_t结构如下：

static ngx_command_t  ngx_epoll_commands[] = {

    {
      /***epoll_events配置项表示epoll_wait函数每次最多返回多少个事件，在ngx_epoll_init函数中
            会预先分配epoll_events配置项指定的epoll_event结构个数**/
      ngx_string("epoll_events"),
      NGX_EVENT_CONF|NGX_CONF_TAKE1,
      ngx_conf_set_num_slot,
      0,
      offsetof(ngx_epoll_conf_t, events),
      NULL },

    {
      /***worker_aio_requests配置项表示创建的aio context能并发处理异步事件的个数，即io_setup函数的第一个参数***/
      ngx_string("worker_aio_requests"),
      NGX_EVENT_CONF|NGX_CONF_TAKE1,
      ngx_conf_set_num_slot,
      0,
      offsetof(ngx_epoll_conf_t, aio_requests),
      NULL },

      ngx_null_command
};

    ngx_epoll_module的ngx_event_module_t结构如下：

ngx_event_module_t  ngx_epoll_module_ctx = {
    &epoll_name,
    ngx_epoll_create_conf,               /* create configuration */
    ngx_epoll_init_conf,                 /* init configuration */

    {
        //向epoll中添加事件时调用
        ngx_epoll_add_event,             /* add an event */
        //从epoll中删除事件时调用
        ngx_epoll_del_event,             /* delete an event */
        /***epoll中不存在enable/disable事件的情况，这里默认设置成添加/删除事件的函数***/
        ngx_epoll_add_event,             /* enable an event */
        ngx_epoll_del_event,             /* disable an event */
        //向epoll中添加tcp连接时调用，每个tcp连接对象一个读事件和一个写事件
        ngx_epoll_add_connection,        /* add an connection */
        //从epoll中删除事件时调用
        ngx_epoll_del_connection,        /* delete an connection */
        NULL,                            /* process the changes */
        // epoll 事件处理函数
        ngx_epoll_process_events,        /* process the events */
        //epoll模块初始化函数
        ngx_epoll_init,                  /* init the events */
        //epoll模块清理函数只在多线程模型中被调用
        ngx_epoll_done,                  /* done the events */
    }
};

    ngx_epoll_create_conf在配置项解析前调用用来初始化配置结构，ngx_epoll_init_conf函数在配置项解析完后调用，如果配置文件是不存在epoll_events或worker_aio_requests配置项，默认将epoll_events设置为512，worker_aio_requests设置为32。ngx_epoll_module_ctx结构体中后十个函数对应于ngx_event_actions_t结构，它是事件模块独有的结构。ngx_epoll_init函数在什么时候被调用呢，它在nginx启动过程中每个worker进程启动后被调用(由ngx_event_core_module的ngx_event_process_init函数调用)。

ngx_epoll_module源码分析

ngx_epoll_init函数：

static ngx_int_t
ngx_epoll_init(ngx_cycle_t *cycle, ngx_msec_t timer)
{
    ngx_epoll_conf_t  *epcf;

    // 获取ngx_epoll_module模块存放配置项的结构
    epcf = ngx_event_get_conf(cycle->conf_ctx, ngx_epoll_module);

    if (ep == -1) {
        // 创建epoll，成功返回描述符，失败返回-1
        ep = epoll_create(cycle->connection_n / 2);

        if (ep == -1) {
            ngx_log_error(NGX_LOG_EMERG, cycle->log, ngx_errno,
                          "epoll_create() failed");
            return NGX_ERROR;
        }

/***如果系统支持aio , 这里初始化aio***/
#if (NGX_HAVE_FILE_AIO)
        ngx_epoll_aio_init(cycle, epcf);

#endif
    }

    /***预分配events个epoll_event结构, epcf->events由epoll_events配置项指定，默认为512***/
    if (nevents < epcf->events) {
        if (event_list) {
            ngx_free(event_list);
        }

        event_list = ngx_alloc(sizeof(struct epoll_event) * epcf->events,
                               cycle->log);
        if (event_list == NULL) {
            return NGX_ERROR;
        }
    }

    nevents = epcf->events;

    //指定I/O读写的方法
    ngx_io = ngx_os_io;

    // 设置ngx_event_actions接口，后续通过ngx_event_actions来调用epoll模块中的方法
    ngx_event_actions = ngx_epoll_module_ctx.actions;

    /***nginx使用epoll事件模型时NGX_HAVE_CLEAR_EVENT宏被定义, NGX_USE_CLEAR_EVENT宏表示使用epoll的ET模式***/
#if (NGX_HAVE_CLEAR_EVENT)
    ngx_event_flags = NGX_USE_CLEAR_EVENT
#else
    ngx_event_flags = NGX_USE_LEVEL_EVENT
#endif
                      |NGX_USE_GREEDY_EVENT
                      |NGX_USE_EPOLL_EVENT;

    return NGX_OK;
}

ngx_epoll_add_event函数：

static ngx_int_t
ngx_epoll_add_event(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags)
{
    int                  op;
    uint32_t             events, prev;
    ngx_event_t         *e;
    ngx_connection_t    *c;
    struct epoll_event   ee;

    //获取事件关联的连接
    c = ev->data;

    events = (uint32_t) event;

    /***根据event参数判断当前是添加读事件还是写事件***/
    if (event == NGX_READ_EVENT) {
        e = c->write;
        prev = EPOLLOUT;
#if (NGX_READ_EVENT != EPOLLIN|EPOLLRDHUP)
        events = EPOLLIN|EPOLLRDHUP;
#endif

    } else {
        e = c->read;
        prev = EPOLLIN|EPOLLRDHUP;
#if (NGX_WRITE_EVENT != EPOLLOUT)
        events = EPOLLOUT;
#endif
    }

    /***如果当前需添加读事件，就通过active标识判断读事件所关联的连接对应的写事件是否活跃(
        活跃表示事件已添加到epoll中)。***/
    if (e->active) {
        op = EPOLL_CTL_MOD;
        events |= prev;

    } else {
        op = EPOLL_CTL_ADD;
    }

    //将flags参数加入到epoll标志中
    ee.events = events | (uint32_t) flags;
    /*** ptr存储事件关联的连接对象(ngx_connection_t*)及事件过期比特位，
            linux平台中任何对象的地址最低位必定为零***/
    ee.data.ptr = (void *) ((uintptr_t) c | ev->instance);

    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, ev->log, 0,
                   "epoll add event: fd:%d op:%d ev:%08XD",
                   c->fd, op, ee.events);

    //向epoll中添加事件
    if (epoll_ctl(ep, op, c->fd, &ee) == -1) {
        ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_errno,
                      "epoll_ctl(%d, %d) failed", op, c->fd);
        return NGX_ERROR;
    }

    //标识事件活跃
    ev->active = 1;
#if 0
    ev->oneshot = (flags & NGX_ONESHOT_EVENT) ? 1 : 0;
#endif

    return NGX_OK;
}

ngx_epoll_del_event函数：

static ngx_int_t
ngx_epoll_del_event(ngx_event_t *ev, ngx_int_t event, ngx_uint_t flags)
{
    int                  op;
    uint32_t             prev;
    ngx_event_t         *e;
    ngx_connection_t    *c;
    struct epoll_event   ee;

    /*
     * when the file descriptor is closed, the epoll automatically deletes
     * it from its queue, so we do not need to delete explicitly the event
     * before the closing the file descriptor
     */

    /***上面的注释说得很清楚了，当文件描述符被关闭后，epoll会自动将其删除。***/
    if (flags & NGX_CLOSE_EVENT) {
        ev->active = 0;
        return NGX_OK;
    }

    //获取事件关联的连接
    c = ev->data;

    /***根据event参数判断当前是删除读事件还是写事件***/
    if (event == NGX_READ_EVENT) {
        e = c->write;
        prev = EPOLLOUT;

    } else {
        e = c->read;
        prev = EPOLLIN|EPOLLRDHUP;
    }

    /***参考ngx_epoll_add_event函数***/
    if (e->active) {
        op = EPOLL_CTL_MOD;
        ee.events = prev | (uint32_t) flags;
        ee.data.ptr = (void *) ((uintptr_t) c | ev->instance);

    } else {
        op = EPOLL_CTL_DEL;
        ee.events = 0;
        ee.data.ptr = NULL;
    }

    ngx_log_debug3(NGX_LOG_DEBUG_EVENT, ev->log, 0,
                   "epoll del event: fd:%d op:%d ev:%08XD",
                   c->fd, op, ee.events);

    //从epoll中删除事件
    if (epoll_ctl(ep, op, c->fd, &ee) == -1) {
        ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_errno,
                      "epoll_ctl(%d, %d) failed", op, c->fd);
        return NGX_ERROR;
    }

    //清除事件活跃标识 
    ev->active = 0;

    return NGX_OK;
}

ngx_epoll_add_connection及ngx_epoll_del_connection函数

    这两个函数的实现很简单，也是通过调用epoll_ctl添加事件，只是会同时将读/写事件一起添加进epoll，这里不再列出源码。

ngx_epoll_process_events函数：

static ngx_int_t
ngx_epoll_process_events(ngx_cycle_t *cycle, ngx_msec_t timer, ngx_uint_t flags)
{
    int                events;
    uint32_t           revents;
    ngx_int_t          instance, i;
    ngx_uint_t         level;
    ngx_err_t          err;
    ngx_event_t       *rev, *wev, **queue;
    ngx_connection_t  *c;

    /* NGX_TIMER_INFINITE == INFTIM */

    ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
                   "epoll timer: %M", timer);

    //调用epoll_wait获取已准备就绪的事件
    events = epoll_wait(ep, event_list, (int) nevents, timer);

    err = (events == -1) ? ngx_errno : 0;

    /***NGX_UPDATE_TIME标识在没有设置timer_resolution配置项时有效表示每次调用epoll_wait函数返回会都更新时间。
            ngx_event_timer_alarm变量在设置timer_resolution配置项时有效，每间隔timer_resolution配置项参数值就会设置
            ngx_event_timer_alarm变量为1表示需更新时间。***/
    if (flags & NGX_UPDATE_TIME || ngx_event_timer_alarm) {
        ngx_time_update();
    }

    //err为非零指示epoll_wait失败
    if (err) {
        if (err == NGX_EINTR) {

            if (ngx_event_timer_alarm) {
                ngx_event_timer_alarm = 0;
                return NGX_OK;
            }

            level = NGX_LOG_INFO;

        } else {
            level = NGX_LOG_ALERT;
        }

        ngx_log_error(level, cycle->log, err, "epoll_wait() failed");
        return NGX_ERROR;
    }

    if (events == 0) {
        if (timer != NGX_TIMER_INFINITE) {
            return NGX_OK;
        }

        ngx_log_error(NGX_LOG_ALERT, cycle->log, 0,
                      "epoll_wait() returned no events without timeout");
        return NGX_ERROR;
    }

    //仅在多线程环境下此锁才有效
    ngx_mutex_lock(ngx_posted_events_mutex);

    /***循环处理已就绪的事件***/
    for (i = 0; i < events; i++) {
        //获取事件关联的连接对象，对象地址最低位保存有在事件添加时设置的事件过期位
        c = event_list[i].data.ptr;

        //取事件过期位
        instance = (uintptr_t) c & 1;
        //屏蔽掉连接对象的最低位
        c = (ngx_connection_t *) ((uintptr_t) c & (uintptr_t) ~1);

        rev = c->read;

        /***同一条连接的读/写事件的instance位值相同，由于下面先处理读事件这里通过读事件
                的过期位来判断连接是否过期，当fd为-1时也表示连接过期。***/
        if (c->fd == -1 || rev->instance != instance) {

            /*
             * the stale event from a file descriptor
             * that was just closed in this iteration
             */

            ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
                           "epoll: stale event %p", c);
            continue;
        }

        //获取连接已就绪的事件类型
        revents = event_list[i].events;

        ngx_log_debug3(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
                       "epoll: fd:%d ev:%04XD d:%p",
                       c->fd, revents, event_list[i].data.ptr);

        /***连接出现错误,EPOLLHUP标识表示收到RST报文。检测到这两种类型时 tcp连接中可能还有
                数据未被读取***/
        if (revents & (EPOLLERR|EPOLLHUP)) {
            ngx_log_debug2(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
                           "epoll_wait() error on fd:%d ev:%04XD",
                           c->fd, revents);
        }

#if 0
        if (revents & ~(EPOLLIN|EPOLLOUT|EPOLLERR|EPOLLHUP)) {
            ngx_log_error(NGX_LOG_ALERT, cycle->log, 0,
                          "strange epoll_wait() events fd:%d ev:%04XD",
                          c->fd, revents);
        }
#endif
        /***如果连接发生错误但未置EPOLLIN及EPOLLOUT，这时我们加上EPOLLIN和EPOLLOUT，在调用读/写事件的
                回调函数时就会知道为什么出现错误。 如果不加EPOLLIN和EPOLLOUT，后面就没法调用读/写事件的
                回调函数也就无法处理该连接了。***/
        if ((revents & (EPOLLERR|EPOLLHUP))
             && (revents & (EPOLLIN|EPOLLOUT)) == 0)
        {
            /*
             * if the error events were returned without EPOLLIN or EPOLLOUT,
             * then add these flags to handle the events at least in one
             * active handler
             */

            revents |= EPOLLIN|EPOLLOUT;
        }

        /***连接可读且活跃***/
        if ((revents & EPOLLIN) && rev->active) {

#if (NGX_HAVE_EPOLLRDHUP)
            //EPOLLRDHUP表示连接对方关闭了读端
            if (revents & EPOLLRDHUP) {
                rev->pending_eof = 1;
            }
#endif

            //NGX_POST_THREAD_EVENTS宏末被使用
            if ((flags & NGX_POST_THREAD_EVENTS) && !rev->accept) {
                rev->posted_ready = 1;

            } else {
                //标识事件已就绪
                rev->ready = 1;
            }

            /***NGX_POST_EVENTS表示事件需要延后处理，这里根据accept标识位将事件加入到相应队列中***/
            if (flags & NGX_POST_EVENTS) {
                queue = (ngx_event_t **) (rev->accept ?
                               &ngx_posted_accept_events : &ngx_posted_events);

                ngx_locked_post_event(rev, queue);

            } else {
                //调用事件的回调函数
                rev->handler(rev);
            }
        }

        wev = c->write;

        /***连接可写且活跃***/
        if ((revents & EPOLLOUT) && wev->active) {

            //重新检查事件是否过期，因为在处理读事件过程中该事件可能已结束。
            if (c->fd == -1 || wev->instance != instance) {

                /*
                 * the stale event from a file descriptor
                 * that was just closed in this iteration
                 */

                ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
                               "epoll: stale event %p", c);
                continue;
            }

            if (flags & NGX_POST_THREAD_EVENTS) {
                wev->posted_ready = 1;

            } else {
                wev->ready = 1;
            }

            if (flags & NGX_POST_EVENTS) {
                ngx_locked_post_event(wev, &ngx_posted_events);

            } else {
                wev->handler(wev);
            }
        }
    }

    ngx_mutex_unlock(ngx_posted_events_mutex);

    return NGX_OK;
}