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);
}
}
}
}