在Redis系统中也存在后台服务的概念，background Service，后台线程在Redis中的表现主要为background I/O Service，有了后台线程的支持，系统在执行的效率上也势必会有不一样的提高。在Redis代码中，描述了此功能的文件为bio.c，同样借此机会学习一下，在C语言中的多线程编程到底是怎么一回事。我们先来看看，在Redis中的background job的工作形式；

/* Background I/O service for Redis.
 *
 * 后台I/O服务
 * This file implements operations that we need to perform in the background.
 * Currently there is only a single operation, that is a background close(2)
 * system call. This is needed as when the process is the last owner of a
 * reference to a file closing it means unlinking it, and the deletion of the
 * file is slow, blocking the server.
 *
 * In the future we'll either continue implementing new things we need or
 * we'll switch to libeio. However there are probably long term uses for this
 * file as we may want to put here Redis specific background tasks (for instance
 * it is not impossible that we'll need a non blocking FLUSHDB/FLUSHALL
 * implementation).
 *
 * DESIGN
 * ------
 *
 * The design is trivial, we have a structure representing a job to perform
 * and a different thread and job queue for every job type.
 * Every thread wait for new jobs in its queue, and process every job
 * sequentially.
 *
 * Jobs of the same type are guaranteed to be processed from the least
 * recently inserted to the most recently inserted (older jobs processed
 * first).
 *
 * Currently there is no way for the creator of the job to be notified about
 * the completion of the operation, this will only be added when/if needed.
 *
 * 作者定义了一个结构体代表一个工作，每个线程等待从相应的job Type工作队列中获取一个job，每个job的排列的都按照时间
 * 有序排列的
 * ----------------------------------------------------------------------------

/* Background job opcodes */
/* 定义了2种后台工作的类别 */
#define REDIS_BIO_CLOSE_FILE    0 /* Deferred close(2) syscall.文件的关闭 */
#define REDIS_BIO_AOF_FSYNC     1 /* Deferred AOF fsync.AOF文件的同步 */ 
/* BIO后台操作类型总数为2个 */
#define REDIS_BIO_NUM_OPS       2

static pthread_t bio_threads[REDIS_BIO_NUM_OPS]; /* 定义了bio线程组变量 */
static pthread_mutex_t bio_mutex[REDIS_BIO_NUM_OPS]; /* 线程相对应的mutex变量，用于同步操作 */
static pthread_cond_t bio_condvar[REDIS_BIO_NUM_OPS];
static list *bio_jobs[REDIS_BIO_NUM_OPS]; /* 每种job类型都是一个列表 */
/* The following array is used to hold the number of pending jobs for every
 * OP type. This allows us to export the bioPendingJobsOfType() API that is
 * useful when the main thread wants to perform some operation that may involve
 * objects shared with the background thread. The main thread will just wait
 * that there are no longer jobs of this type to be executed before performing
 * the sensible operation. This data is also useful for reporting. */
static unsigned long long bio_pending[REDIS_BIO_NUM_OPS];   /* 此类型job等待执行的数量 */

/* This structure represents a background Job. It is only used locally to this
 * file as the API does not expose the internals at all. */
/* background Job结构体 */
struct bio_job {
	//job创建的时间
    time_t time; /* Time at which the job was created. */
    /* Job specific arguments pointers. If we need to pass more than three
     * arguments we can just pass a pointer to a structure or alike. */
    /* job特定参数指针 */
    void *arg1, *arg2, *arg3;
};

/* Exported API */
void bioInit(void); /* background I/O初始化操作 */
void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3); /* 创建后台job,通过传入的3个参数初始化 */
unsigned long long bioPendingJobsOfType(int type); /* 返回type类型的job正在等待被执行的个数 */
void bioWaitPendingJobsLE(int type, unsigned long long num); /* 返回type类型的job正在等待被执行的个数 */
time_t bioOlderJobOfType(int type); 
void bioKillThreads(void); /* 杀死后台所有线程 */

/* Initialize the background system, spawning the thread. */
/* background I/O初始化操作 */
void bioInit(void) {
    pthread_attr_t attr;
    pthread_t thread;
    size_t stacksize;
    int j;

    /* Initialization of state vars and objects */
    for (j = 0; j < REDIS_BIO_NUM_OPS; j++) {
        pthread_mutex_init(&bio_mutex[j],NULL);
        pthread_cond_init(&bio_condvar[j],NULL);
        //创建每个job类型的List列表
        bio_jobs[j] = listCreate();
        bio_pending[j] = 0;
    }

    /* Set the stack size as by default it may be small in some system */
    //设置线程栈空间
    pthread_attr_init(&attr);
    pthread_attr_getstacksize(&attr,&stacksize);
    if (!stacksize) stacksize = 1; /* The world is full of Solaris Fixes */
    while (stacksize < REDIS_THREAD_STACK_SIZE) stacksize *= 2;
    pthread_attr_setstacksize(&attr, stacksize);

    /* Ready to spawn our threads. We use the single argument the thread
     * function accepts in order to pass the job ID the thread is
     * responsible of. */
    for (j = 0; j < REDIS_BIO_NUM_OPS; j++) {
        void *arg = (void*)(unsigned long) j;
        //创建2个线程，专门运行相应类型的job
        if (pthread_create(&thread,&attr,bioProcessBackgroundJobs,arg) != 0) {
            redisLog(REDIS_WARNING,"Fatal: Can't initialize Background Jobs.");
            exit(1);
        }
        //赋值到相应的Thread中
        bio_threads[j] = thread;
    }
}

/* 创建后台job,通过传入的3个参数初始化 */
void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3) {
    struct bio_job *job = zmalloc(sizeof(*job));

    job->time = time(NULL);
    job->arg1 = arg1;
    job->arg2 = arg2;
    job->arg3 = arg3;
    pthread_mutex_lock(&bio_mutex[type]);
    //加入相对应的job type列表
    listAddNodeTail(bio_jobs[type],job);
    //等待的job数量增加1
    bio_pending[type]++;
    pthread_cond_signal(&bio_condvar[type]);
    pthread_mutex_unlock(&bio_mutex[type]);
}

/* 执行后台的job,参数内包含着哪种type */
void *bioProcessBackgroundJobs(void *arg) {
   	......
    while(1) {
        listNode *ln;

        /* The loop always starts with the lock hold. */
        if (listLength(bio_jobs[type]) == 0) {
            pthread_cond_wait(&bio_condvar[type],&bio_mutex[type]);
            continue;
        }
        /* Pop the job from the queue. */
        //从工作列表中取出第一个job
        ln = listFirst(bio_jobs[type]);
        job = ln->value;
        /* It is now possible to unlock the background system as we know have
         * a stand alone job structure to process.*/
        pthread_mutex_unlock(&bio_mutex[type]);

        /* Process the job accordingly to its type. */
        //执行具体的工作
        if (type == REDIS_BIO_CLOSE_FILE) {
            close((long)job->arg1);
        } else if (type == REDIS_BIO_AOF_FSYNC) {
            aof_fsync((long)job->arg1);
        } else {
            redisPanic("Wrong job type in bioProcessBackgroundJobs().");
        }
        zfree(job);

        /* Lock again before reiterating the loop, if there are no longer
         * jobs to process we'll block again in pthread_cond_wait(). */
        pthread_mutex_lock(&bio_mutex[type]);
        listDelNode(bio_jobs[type],ln);
        bio_pending[type]--;
    }
}

/* Kill the running bio threads in an unclean way. This function should be
 * used only when it's critical to stop the threads for some reason.
 * Currently Redis does this only on crash (for instance on SIGSEGV) in order
 * to perform a fast memory check without other threads messing with memory. */
/* 杀死后台所有线程 */
void bioKillThreads(void) {
    int err, j;

    for (j = 0; j < REDIS_BIO_NUM_OPS; j++) {
    	//调用pthread_cancel方法kill当前的后台线程
        if (pthread_cancel(bio_threads[j]) == 0) {
            if ((err = pthread_join(bio_threads[j],NULL)) != 0) {
                redisLog(REDIS_WARNING,
                    "Bio thread for job type #%d can be joined: %s",
                        j, strerror(err));
            } else {
                redisLog(REDIS_WARNING,
                    "Bio thread for job type #%d terminated",j);
            }
        }
    }
}

Redis源码分析（二十九）--- bio后台I/O服务的实现