Redis源码分析（二十九）--- bio后台I/O服务的实现

         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 * ----------------------------------------------------------------------------

这里总共与2种Background I/O Type:

/* 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

一个是AOF文件的同步操作，AOF就是“Append ONLY File”的缩写，记录每次的数据改变的写操作，用于数据的恢复。还有一个我好像没碰到过，CLOSE FILE，难道是异步关闭文件的意思。

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;};

上面声明了一些变量，包括bio_threads线程数组，总数2个，bio_jobs列表数组，存放每种Type的job。下面我们看主要的一些方法:

/* 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 */

首先看初始化操作；

/* 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;    }}

也就是说，执行完上述的操作之后，在bio_threads线程中就运行着2个线程，从各自的job列表中取出相应的等待执行的jo；

/* 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]);}

简洁的创建background job操作,上面利用了mutex变量实现了线程同步操作，保证线程安全。下面看一下最重要的执行background Job的操作实现（省略了部分代码）:

/* 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]--;    }}

while循环，从队列中取出一个，执行一个操作。当然，如果想马上停止一切后台线程，可以执行下面的方法，调用
pthread_cancel:

/* 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);            }        }    }}