memcached的通信机制

memcached的通信机制是有一个主线程负责监听来的请求，然后main thread负责将得到的请求分发到各个worker thread中。
每个worker thread维护一个请求队列。
主线程通过如下方式与worker 线程交互：
与线程相关的核心代码都在thread.c当中。

/*
 * Dispatches a new connection to another thread. This is only ever called
 * from the main thread, either during initialization (for UDP) or because
 * of an incoming connection.
 */
void dispatch_conn_new(int sfd, enum conn_states init_state, int event_flags,
                       int read_buffer_size, enum network_transport transport) {
    CQ_ITEM *item = cqi_new();
    char buf[1];
    int tid = (last_thread + 1) % settings.num_threads;

    LIBEVENT_THREAD *thread = threads + tid;

    last_thread = tid;

    item->sfd = sfd;
    item->init_state = init_state;
    item->event_flags = event_flags;
    item->read_buffer_size = read_buffer_size;
    item->transport = transport;

    cq_push(thread->new_conn_queue, item);

    MEMCACHED_CONN_DISPATCH(sfd, thread->thread_id);
    buf[0] = 'c';
    if (write(thread->notify_send_fd, buf, 1) != 1) {
        perror("Writing to thread notify pipe");
    }
}

主线程通过轮询将请求发送到相应的线程中。threads是一个全局的变量，cq_push将写请求放入到该线程的队列中。向thread的管道中写入一个字节的内容，通知工作线程，处理自己的线程队列里面的任务。