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); } } } } ~~~