Redis除了主线程外，还有三个后台线程，在Redis启动时，会调用InitServerLast函数在基本初始化完成之后，做最后的初始化：
void InitServerLast() {
bioInit();
server.initial_memory_usage = zmalloc_used_memory();
}
bioInit就是用来初始化后台的三个线程
后台线程实现
后台线程的实现在bio.h/c中，有三种任务类型，也就对应了三个后台线程：
/* 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
BIO_CLOSE_FILE：关闭文件
BIO_AOF_FSYNC：异步fsync持久化aof文件
BIO_LAZY_FREE：KV对的内存懒释放
这三种任务都是比较耗时的任务，同时对于redisDb的正确性没有影响，可以从主线程中剥离出来，交给后台线程执行，避免主线程的阻塞，造成抖动。
在bioInit中，初始化这三个后台任务线程的锁、条件变量、任务队列：
void bioInit(void) {
pthread_attr_t attr;
pthread_t thread;
size_t stacksize;
int j;
/* Initialization of state vars and objects */
// 初始化任务线程的锁、条件变量、任务队列
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. */
// 创建并启动后台线程
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;
}
}
线程的执行函数为bioProcessBackgroundJobs，不停的从线程任务队列中取出任务，根据任务类型，调用不同的函数处理：
void *bioProcessBackgroundJobs(void *arg) {
...
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. */
// 根据任务类型，调用不同的函数处理
if (type == BIO_CLOSE_FILE) {
close((long)job->arg1);
} else if (type == BIO_AOF_FSYNC) {
redis_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);
/* 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]--;
/* Unblock threads blocked on bioWaitStepOfType() if any. */
pthread_cond_broadcast(&bio_step_cond[type]);
}
}
其中lazy free分成三种：
lazyfreeFreeObjectFromBioThread：lazy free一对KV
lazyfreeFreeDatabaseFromBioThread：lazy free一个redisDb
lazyfreeFreeSlotsMapFromBioThread：lazy free一个slot中的所有key
后台任务
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;
};
后台任务可能包含三个参数，用于后台线程执行
后台任务的创建和提交很简单，初始化一个后台任务并复制，然后加锁提交到后台线程的任务列表中
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]);
}
后台任务的提交时机主要有以下几种：
1、 删除（包括用户删除、过期健清理、内存淘汰）一个key时，如果设置了可以lazyfree，同时value较大，会提交一个lazyfreekey的任务；
2、 清空一个DB时，会提交一个lazyfreeDB/Slot的任务；
3、 开启aof时，并且可以异步flush，则会提交一个异步flushaof任务；
4、 开启aof时，aofrewrite或者切换时，会提交一个异步close文件任务；