簡介

Redis Bio 就是Background I/O service for Redis.
他把那些耗時的io操作放到后臺的線程來執(zhí)行。主線程就專注服務(wù)。避免那些耗時的釋放，寫入操作等造成服務(wù)的等待。
實(shí)現(xiàn)也特別的簡單。采用新建處理線程。使用臨界區(qū)投放任務(wù)的方式完成操作。

核心的一些定義和方法

static pthread_t bio_threads[BIO_NUM_OPS]; //保存pthread
static pthread_mutex_t bio_mutex[BIO_NUM_OPS];//保存mutex
static pthread_cond_t bio_newjob_cond[BIO_NUM_OPS];//保存新任務(wù)條件變量
static pthread_cond_t bio_step_cond[BIO_NUM_OPS];//保存step條件變量
static list *bio_jobs[BIO_NUM_OPS];//保存joblist

static unsigned long long bio_pending[BIO_NUM_OPS];//保存pening數(shù)量

struct bio_job {
    time_t time; /* Time at which the job was created. */ //創(chuàng)建時間
    /* Job specific arguments pointers. If we need to pass more than three
     * arguments we can just pass a pointer to a structure or alike. */
    void *arg1, *arg2, *arg3;//三個保存的參數(shù)
};

初始化bio

/* Initialize the background system, spawning the thread. */
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 < BIO_NUM_OPS; j++) {//基本的初始化操作
        pthread_mutex_init(&bio_mutex[j],NULL);
        pthread_cond_init(&bio_newjob_cond[j],NULL);
        pthread_cond_init(&bio_step_cond[j],NULL);
        bio_jobs[j] = listCreate();//創(chuàng)建任務(wù)隊(duì)列 
        bio_pending[j] = 0;//等待數(shù)量是0
    }

    /* Set the stack size as by default it may be small in some system */
    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);//stack size

    /* 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 < BIO_NUM_OPS; j++) {
        void *arg = (void*)(unsigned long) j;
        if (pthread_create(&thread,&attr,bioProcessBackgroundJobs,arg) != 0) {//創(chuàng)建線程
            serverLog(LL_WARNING,"Fatal: Can't initialize Background Jobs.");
            exit(1);
        }
        bio_threads[j] = thread;
    }
}

整個創(chuàng)建過程是一個標(biāo)準(zhǔn)的初始化過程。沒有什么特別的地方。

線程函數(shù)

void *bioProcessBackgroundJobs(void *arg) {//線程入口
    struct bio_job *job;
    unsigned long type = (unsigned long) arg;
    sigset_t sigset;

    /* Check that the type is within the right interval. */
    if (type >= BIO_NUM_OPS) {//type不對 
        serverLog(LL_WARNING,
            "Warning: bio thread started with wrong type %lu",type);
        return NULL;
    }

    /* Make the thread killable at any time, so that bioKillThreads()
     * can work reliably. */
    pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);//設(shè)置響應(yīng)cancel
    pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);//設(shè)置立即結(jié)束

    pthread_mutex_lock(&bio_mutex[type]);//鎖住互斥
    /* Block SIGALRM so we are sure that only the main thread will
     * receive the watchdog signal. */
    sigemptyset(&sigset);
    sigaddset(&sigset, SIGALRM);
    if (pthread_sigmask(SIG_BLOCK, &sigset, NULL))//只讓主線程處理 SIGALRM
        serverLog(LL_WARNING,
            "Warning: can't mask SIGALRM in bio.c thread: %s", strerror(errno));

    while(1) {
        listNode *ln;

        /* The loop always starts with the lock hold. */
        if (listLength(bio_jobs[type]) == 0) {
            pthread_cond_wait(&bio_newjob_cond[type],&bio_mutex[type]);//釋放mutex 并且進(jìn)入阻塞等待
            continue;//防止意外喚醒
        }
        /* Pop the job from the queue. */
        ln = listFirst(bio_jobs[type]);// 獲取新的任務(wù)
        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]);//釋放鎖可以加入新的任務(wù)

        /* Process the job accordingly to its type. */
        if (type == BIO_CLOSE_FILE) {//判斷自己的類型
            close((long)job->arg1);
        } else if (type == BIO_AOF_FSYNC) {
            aof_fsync((long)job->arg1);
        } else if (type == BIO_LAZY_FREE) {
            /* What we free changes depending on what arguments are set:
             * arg1 -> free the object at pointer.
             * arg2 & arg3 -> free two dictionaries (a Redis DB).
             * only arg3 -> free the skiplist. */
            if (job->arg1)
                lazyfreeFreeObjectFromBioThread(job->arg1);
            else if (job->arg2 && job->arg3)
                lazyfreeFreeDatabaseFromBioThread(job->arg2,job->arg3);
            else if (job->arg3)
                lazyfreeFreeSlotsMapFromBioThread(job->arg3);
        } else {
            serverPanic("Wrong job type in bioProcessBackgroundJobs().");
        }
        zfree(job);

        /* Unblock threads blocked on bioWaitStepOfType() if any. */
        pthread_cond_broadcast(&bio_step_cond[type]);//喚醒所有的 setp_cond

        /* 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]);//鎖住
        listDelNode(bio_jobs[type],ln);//刪除完成的任務(wù)
        bio_pending[type]--;//pending --
    }
}

整理流程：
1獲取鎖，判斷任務(wù)是否存在，不存在則進(jìn)入cond_wait,存在就提取出一個任務(wù)，然后根據(jù)任務(wù)類型，進(jìn)行操作。
2.執(zhí)行結(jié)束后喚醒所有的step_cond，再次獲取鎖。刪除完成的任務(wù)。進(jìn)入循環(huán)

創(chuàng)建一個任務(wù)

void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3) {//創(chuàng)建job
    struct bio_job *job = zmalloc(sizeof(*job));//分配內(nèi)存
    job->time = time(NULL);//開始時間
    job->arg1 = arg1;//保存參數(shù)
    job->arg2 = arg2;
    job->arg3 = arg3;
    pthread_mutex_lock(&bio_mutex[type]);//獲取鎖
    listAddNodeTail(bio_jobs[type],job);//插入任務(wù)
    bio_pending[type]++;//pending++
    pthread_cond_signal(&bio_newjob_cond[type]);//通知等待
    pthread_mutex_unlock(&bio_mutex[type]);
}

關(guān)閉后臺io線程

void bioKillThreads(void) {
    int err, j;

    for (j = 0; j < BIO_NUM_OPS; j++) {
        if (pthread_cancel(bio_threads[j]) == 0) {//發(fā)送cancle
            if ((err = pthread_join(bio_threads[j],NULL)) != 0) {//join等待
                serverLog(LL_WARNING,
                    "Bio thread for job type #%d can be joined: %s",
                        j, strerror(err));
            } else {
                serverLog(LL_WARNING,
                    "Bio thread for job type #%d terminated",j);
            }
        }
    }
}

一些狀態(tài)的獲取

unsigned long long bioWaitStepOfType(int type) {
    unsigned long long val;
    pthread_mutex_lock(&bio_mutex[type]);//加鎖
    val = bio_pending[type];
    if (val != 0) {//
        pthread_cond_wait(&bio_step_cond[type],&bio_mutex[type]);//d等待通知
        val = bio_pending[type];
    }
    pthread_mutex_unlock(&bio_mutex[type]);//釋放鎖
    return val;
}
unsigned long long bioPendingJobsOfType(int type) {
    unsigned long long val;
    pthread_mutex_lock(&bio_mutex[type]);
    val = bio_pending[type];
    pthread_mutex_unlock(&bio_mutex[type]);
    return val;
}