理解Redis 6 的多线程

2020.5.2日，Redis 6.0.1正式发布。除了增加新功能和新的API，支持多线程是最大变化。本文以简化的方式理解Redis线程模型的演进。找我交流

Redis单线程

严格讲，Redis 并不是单线程。有后台线程在工作，处理一些较为缓慢的操作，例如无用连接的释放、大key的删除等。client端命令的请求获取 (socket 读)、解析、执行、内容返回 (socket 写) 都是由一个线程处理，因此我们常说的“单线程”指的是处理核心处理的线程只有一个。

处理流程

如下图：

Redis采用evport，epoll，kqueue和select四种方式实现多路复用，增加链接处理，默认是epoll

参考ae.c文件L47

/* Include the best multiplexing layer supported by this system.
 * The following should be ordered by performances, descending. */
#ifdef HAVE_EVPORT
#include "ae_evport.c"
#else
    #ifdef HAVE_EPOLL
    #include "ae_epoll.c"
    #else
        #ifdef HAVE_KQUEUE
        #include "ae_kqueue.c"
        #else
        #include "ae_select.c"
        #endif
    #endif
#endif

优点 VS 缺点

优势：

1、不存在锁的问题

2、避免线程间CPU切换

缺点：

1、单线程无法利用多CPU

2、串行操作，某个操作“出问题”会“阻塞”后续操作

Redis 6 多线程

处理流程

默认并不开启多线程，需要参数设置，IO_THREADS_MAX_NUM 最大为128

network.c文件 L2886

/* Initialize the data structures needed for threaded I/O. */
void initThreadedIO(void) {
    io_threads_active = 0; /* We start with threads not active. */

    /* Don't spawn any thread if the user selected a single thread:
     * we'll handle I/O directly from the main thread. */
    if (server.io_threads_num == 1) return;

    if (server.io_threads_num > IO_THREADS_MAX_NUM) {
        serverLog(LL_WARNING,"Fatal: too many I/O threads configured. "
                             "The maximum number is %d.", IO_THREADS_MAX_NUM);
        exit(1);
    }

    /* Spawn and initialize the I/O threads. */
    for (int i = 0; i < server.io_threads_num; i++) {
        /* Things we do for all the threads including the main thread. */
        io_threads_list[i] = listCreate();
        if (i == 0) continue; /* Thread 0 is the main thread. */

        /* Things we do only for the additional threads. */
        pthread_t tid;
        pthread_mutex_init(&io_threads_mutex[i],NULL);
        io_threads_pending[i] = 0;
        pthread_mutex_lock(&io_threads_mutex[i]); /* Thread will be stopped. */
        if (pthread_create(&tid,NULL,IOThreadMain,(void*)(long)i) != 0) {
            serverLog(LL_WARNING,"Fatal: Can't initialize IO thread.");
            exit(1);
        }
        io_threads[i] = tid;
    }
}

加入队列

network.c文件L3040

/* Return 1 if we want to handle the client read later using threaded I/O.
 * This is called by the readable handler of the event loop.
 * As a side effect of calling this function the client is put in the
 * pending read clients and flagged as such. */
int postponeClientRead(client *c) {
    if (io_threads_active &&
        server.io_threads_do_reads &&
        !ProcessingEventsWhileBlocked &&
        !(c->flags & (CLIENT_MASTER|CLIENT_SLAVE|CLIENT_PENDING_READ)))
    {
        c->flags |= CLIENT_PENDING_READ;
        listAddNodeHead(server.clients_pending_read,c);
        return 1;
    } else {
        return 0;
    }
}

/* When threaded I/O is also enabled for the reading + parsing side, the
 * readable handler will just put normal clients into a queue of clients to
 * process (instead of serving them synchronously). This function runs
 * the queue using the I/O threads, and process them in order to accumulate
 * the reads in the buffers, and also parse the first command available
 * rendering it in the client structures. */
int handleClientsWithPendingReadsUsingThreads(void) {

基础类图

优点 VS 缺点

优点：

1、提高响应速度，充分使用CPU

缺点：

1、增加了代码复杂性

总结

Redis的多路复用技术，支持epoll、kqueue、selector
5.0版本及以前，处理客户端请求的线程只有一个，串行处理
6.0版本引入了worker Thread，只处理网络IO读取和写入，核心IO负责串行处理客户端指令

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