void
ngx_event_accept(ngx_event_t *ev)
{
 socklen_t  socklen;
 ngx_err_t  err;
 ngx_log_t  *log;
 ngx_uint_t  level;
 ngx_socket_t  s;
 ngx_event_t  *rev, *wev;
 ngx_listening_t  *ls;
 ngx_connection_t *c, *lc;
 ngx_event_conf_t *ecf;
 u_char  sa[NGX_SOCKADDRLEN];
 
 if (ev->timedout) {
  if (ngx_enable_accept_events((ngx_cycle_t *) ngx_cycle) != NGX_OK) {
   return;
  }
 
  ev->timedout = 0;
 }
 
 ecf = ngx_event_get_conf(ngx_cycle->conf_ctx, ngx_event_core_module);
 
 if (ngx_event_flags & NGX_USE_RTSIG_EVENT) {
  ev->available = 1;
 
 } else if (!(ngx_event_flags & NGX_USE_KQUEUE_EVENT)) {
  ev->available = ecf->multi_accept;
 }
 
 lc = ev->data;
 ls = lc->listening;
 ev->ready = 0;
 
 do {
  socklen = NGX_SOCKADDRLEN;
 
  /* 1、accept方法试图建立连接，非阻塞调用 */
  s = accept(lc->fd, (struct sockaddr *) sa, &socklen);
 
  if (s == (ngx_socket_t) -1)
  {
   err = ngx_socket_errno;
 
   if (err == NGX_EAGAIN)
   {
    /* 没有连接，直接返回 */
    return;
   }
 
   level = NGX_LOG_ALERT;
 
   if (err == NGX_ECONNABORTED) {
    level = NGX_LOG_ERR;
 
   } else if (err == NGX_EMFILE || err == NGX_ENFILE) {
    level = NGX_LOG_CRIT;
   }
 
   if (err == NGX_ECONNABORTED) {
    if (ngx_event_flags & NGX_USE_KQUEUE_EVENT) {
     ev->available--;
    }
 
    if (ev->available) {
     continue;
    }
   }
 
   if (err == NGX_EMFILE || err == NGX_ENFILE) {
    if (ngx_disable_accept_events((ngx_cycle_t *) ngx_cycle)
     != NGX_OK)
    {
     return;
    }
 
    if (ngx_use_accept_mutex) {
     if (ngx_accept_mutex_held) {
      ngx_shmtx_unlock(&ngx_accept_mutex);
      ngx_accept_mutex_held = 0;
     }
 
     ngx_accept_disabled = 1;
 
    } else {
     ngx_add_timer(ev, ecf->accept_mutex_delay);
    }
   }
 
   return;
  }
 
  /* 2、设置负载均衡阈值 */
  ngx_accept_disabled = ngx_cycle->connection_n / 8
        - ngx_cycle->free_connection_n;
 
  /* 3、从连接池获得一个连接对象 */
  c = ngx_get_connection(s, ev->log);
 
  /* 4、为连接创建内存池 */
  c->pool = ngx_create_pool(ls->pool_size, ev->log);
 
  c->sockaddr = ngx_palloc(c->pool, socklen);
 
  ngx_memcpy(c->sockaddr, sa, socklen);
 
  log = ngx_palloc(c->pool, sizeof(ngx_log_t));
 
  /* set a blocking mode for aio and non-blocking mode for others */
  /* 5、设置套接字属性为阻塞或非阻塞 */
  if (ngx_inherited_nonblocking) {
   if (ngx_event_flags & NGX_USE_AIO_EVENT) {
    if (ngx_blocking(s) == -1) {
     ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_socket_errno,
         ngx_blocking_n " failed");
     ngx_close_accepted_connection(c);
     return;
    }
   }
 
  } else {
   if (!(ngx_event_flags & (NGX_USE_AIO_EVENT|NGX_USE_RTSIG_EVENT))) {
    if (ngx_nonblocking(s) == -1) {
     ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_socket_errno,
         ngx_nonblocking_n " failed");
     ngx_close_accepted_connection(c);
     return;
    }
   }
  }
 
  *log = ls->log;
 
  c->recv = ngx_recv;
  c->send = ngx_send;
  c->recv_chain = ngx_recv_chain;
  c->send_chain = ngx_send_chain;
 
  c->log = log;
  c->pool->log = log;
 
  c->socklen = socklen;
  c->listening = ls;
  c->local_sockaddr = ls->sockaddr;
  c->local_socklen = ls->socklen;
 
  c->unexpected_eof = 1;
 
  rev = c->read;
  wev = c->write;
 
  wev->ready = 1;
 
  if (ngx_event_flags & (NGX_USE_AIO_EVENT|NGX_USE_RTSIG_EVENT)) {
   /* rtsig, aio, iocp */
   rev->ready = 1;
  }
 
  if (ev->deferred_accept) {
   rev->ready = 1;
 
  }
 
  rev->log = log;
  wev->log = log;
 
  /*
   * TODO: MT: - ngx_atomic_fetch_add()
   *  or protection by critical section or light mutex
   *
   * TODO: MP: - allocated in a shared memory
   *   - ngx_atomic_fetch_add()
   *  or protection by critical section or light mutex
   */
 
  c->number = ngx_atomic_fetch_add(ngx_connection_counter, 1);
 
  if (ls->addr_ntop) {
   c->addr_text.data = ngx_pnalloc(c->pool, ls->addr_text_max_len);
   if (c->addr_text.data == NULL) {
    ngx_close_accepted_connection(c);
    return;
   }
 
   c->addr_text.len = ngx_sock_ntop(c->sockaddr, c->socklen,
            c->addr_text.data,
            ls->addr_text_max_len, 0);
   if (c->addr_text.len == 0) {
    ngx_close_accepted_connection(c);
    return;
   }
  }
 
  /* 6、将新连接对应的读写事件添加到epoll对象中 */
  if (ngx_add_conn && (ngx_event_flags & NGX_USE_EPOLL_EVENT) == 0) {
   if (ngx_add_conn(c) == NGX_ERROR) {
    ngx_close_accepted_connection(c);
    return;
   }
  }
 
  log->data = NULL;
  log->handler = NULL;
 
  /* 7、TCP建立成功调用的方法，这个方法在ngx_listening_t结构体中 */
  ls->handler(c);
 
 } while (ev->available); /* available标志表示一次尽可能多的建立连接，由配置项multi_accept决定 */
}

if (flags & NGX_POST_EVENTS)
{
 /* 延后处理这批事件 */
 queue = (ngx_event_t **) (rev->accept "htmlcode">

if (ngx_accept_disabled > 0)
{
  ngx_accept_disabled--;
}
else
{
 if (ngx_trylock_accept_mutex(cycle) == NGX_ERROR)
 {
  return;
 }
 ....
}



这说明，当某个进程当前的连接数达到能够处理的总连接数的7/8时，负载均衡机制被触发，进程停止响应新连接。
参考：
《深入理解Nginx》 P328-P334.