redis源码阅读之bio

redis虽然整体使用单线程实现，但是为了确保client响应时间，还是将一些比较耗时的操作移到了单个线程里面，在后台运行，这就是我们今天要讲的bio。
首先，在4.0.11当中，bio留给外部调用的接口如下

/* Exported API */
void bioInit(void);
void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3);
unsigned long long bioPendingJobsOfType(int type);
unsigned long long bioWaitStepOfType(int type);
time_t bioOlderJobOfType(int type);
void bioKillThreads(void);

具体要用到bio的，共有三种情况：

/* Background job opcodes */
#define BIO_CLOSE_FILE    0 /* Deferred close(2) syscall. */
#define BIO_AOF_FSYNC     1 /* Deferred AOF fsync. */
#define BIO_LAZY_FREE     2 /* Deferred objects freeing. */
#define BIO_NUM_OPS       3

为了分开执行以上三种情况，redis为每一种情况都构建了如下变量

static pthread_t bio_threads[BIO_NUM_OPS];
static pthread_mutex_t bio_mutex[BIO_NUM_OPS];
static pthread_cond_t bio_newjob_cond[BIO_NUM_OPS];
static pthread_cond_t bio_step_cond[BIO_NUM_OPS];
static list *bio_jobs[BIO_NUM_OPS];

static unsigned long long bio_pending[BIO_NUM_OPS];

最底下的两个分别是每种任务的队列，以及每种任务pending的数量，上面的变量就是为了操作这两个队列的相关线程、互斥量、条件变量了。
后台任务是用如下结构来表示的

/* This structure represents a background Job. It is only used locally to this
 * file as the API does not expose the internals at all. */
struct bio_job {
    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. */
    void *arg1, *arg2, *arg3;
};

其初始化工作就是初始化每一种任务的相关变量，创建线程并等待：

/* 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++) {
        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();
        bio_pending[j] = 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);

    /* 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) {
            serverLog(LL_WARNING,"Fatal: Can't initialize Background Jobs.");
            exit(1);
        }
        bio_threads[j] = thread;
    }
}

每个线程的具体实现如下

void *bioProcessBackgroundJobs(void *arg) {
    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) {
        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);
    pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, NULL);

    pthread_mutex_lock(&bio_mutex[type]);
    /* 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))
        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]);
            continue;
        }
        /* Pop the job from the queue. */
        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. */
        if (type == BIO_CLOSE_FILE) {
            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]);

        /* 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]--;
    }
}

创建具体任务的api就是传入类型和参数，将任务放入队列，然后signal到等待线程

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]);
    listAddNodeTail(bio_jobs[type],job);
    bio_pending[type]++;
    pthread_cond_signal(&bio_newjob_cond[type]);
    pthread_mutex_unlock(&bio_mutex[type]);
}

获取指定类型的pending job 的数量

/* Return the number of pending jobs of the specified type. */
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;
}

bioWaitStepOfType(int type)和上面的非常类似，只不过是在pending为0的时候直接返回，若非0，则等待当前类型的任务的下一个开始处理再返回

unsigned long long bioWaitStepOfType(int type) {
    unsigned long long val;
    pthread_mutex_lock(&bio_mutex[type]);
    val = bio_pending[type];
    if (val != 0) {
        pthread_cond_wait(&bio_step_cond[type],&bio_mutex[type]);
        val = bio_pending[type];
    }
    pthread_mutex_unlock(&bio_mutex[type]);
    return val;
}

杀死所有线程就比较简单了

void bioKillThreads(void) {
    int err, j;

    for (j = 0; j < BIO_NUM_OPS; j++) {
        if (pthread_cancel(bio_threads[j]) == 0) {
            if ((err = pthread_join(bio_threads[j],NULL)) != 0) {
                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);
            }
        }
    }
}