? ? ? ?在Redis系統中也存在后臺服務的概念，background Service，后臺線程在Redis中的表現主要為background I/O Service，有了后臺線程的支持，系統在執行的效率上也勢必會有不一樣的提高。在Redis代碼中，描述了此功能的文件為bio.c，同樣借此機會學習一下，在C語言中的多線程編程到底是怎么一回事。我們先來看看，在Redis中的background job的工作形式； ~~~ /* Background I/O service for Redis. * * 后臺I/O服務 * 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. * * 作者定義了一個結構體代表一個工作，每個線程等待從相應的job Type工作隊列中獲取一個job，每個job的排列的都按照時間 * 有序排列的 * ---------------------------------------------------------------------------- ~~~ 這里總共與2種Background I/O Type: ~~~ /* Background job opcodes */ /* 定義了2種后臺工作的類別 */ #define REDIS_BIO_CLOSE_FILE 0 /* Deferred close(2) syscall.文件的關閉 */ #define REDIS_BIO_AOF_FSYNC 1 /* Deferred AOF fsync.AOF文件的同步 */ /* BIO后臺操作類型總數為2個 */ #define REDIS_BIO_NUM_OPS 2 ~~~ 一個是AOF文件的同步操作，AOF就是“Append ONLY File”的縮寫，記錄每次的數據改變的寫操作，用于數據的恢復。還有一個我好像沒碰到過，CLOSE FILE，難道是異步關閉文件的意思。 ~~~ static pthread_t bio_threads[REDIS_BIO_NUM_OPS]; /* 定義了bio線程組變量 */ static pthread_mutex_t bio_mutex[REDIS_BIO_NUM_OPS]; /* 線程相對應的mutex變量，用于同步操作 */ static pthread_cond_t bio_condvar[REDIS_BIO_NUM_OPS]; static list *bio_jobs[REDIS_BIO_NUM_OPS]; /* 每種job類型都是一個列表 */ /* 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]; /* 此類型job等待執行的數量 */ /* 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結構體 */ struct bio_job { //job創建的時間 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特定參數指針 */ void *arg1, *arg2, *arg3; }; ~~~ 上面聲明了一些變量，包括bio_threads線程數組，總數2個，bio_jobs列表數組，存放每種Type的job。下面我們看主要的一些方法: ~~~ /* Exported API */ void bioInit(void); /* background I/O初始化操作 */ void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3); /* 創建后臺job,通過傳入的3個參數初始化 */ unsigned long long bioPendingJobsOfType(int type); /* 返回type類型的job正在等待被執行的個數 */ void bioWaitPendingJobsLE(int type, unsigned long long num); /* 返回type類型的job正在等待被執行的個數 */ time_t bioOlderJobOfType(int type); void bioKillThreads(void); /* 殺死后臺所有線程 */ ~~~ 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); //創建每個job類型的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 */ //設置線程棧空間 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; //創建2個線程，專門運行相應類型的job if (pthread_create(&thread,&attr,bioProcessBackgroundJobs,arg) != 0) { redisLog(REDIS_WARNING,"Fatal: Can't initialize Background Jobs."); exit(1); } //賦值到相應的Thread中 bio_threads[j] = thread; } } ~~~ 也就是說，執行完上述的操作之后，在bio_threads線程中就運行著2個線程，從各自的job列表中取出相應的等待執行的jo； ~~~ /* 創建后臺job,通過傳入的3個參數初始化 */ 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]); //加入相對應的job type列表 listAddNodeTail(bio_jobs[type],job); //等待的job數量增加1 bio_pending[type]++; pthread_cond_signal(&bio_condvar[type]); pthread_mutex_unlock(&bio_mutex[type]); } ~~~ 簡潔的創建background job操作,上面利用了mutex變量實現了線程同步操作，保證線程安全。下面看一下最重要的執行background Job的操作實現（省略了部分代碼）: ~~~ /* 執行后臺的job,參數內包含著哪種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. */ //從工作列表中取出第一個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. */ //執行具體的工作 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循環，從隊列中取出一個，執行一個操作。當然，如果想馬上停止一切后臺線程，可以執行下面的方法，調用 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. */ /* 殺死后臺所有線程 */ void bioKillThreads(void) { int err, j; for (j = 0; j < REDIS_BIO_NUM_OPS; j++) { //調用pthread_cancel方法kill當前的后臺線程 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); } } } } ~~~