redis7.x源码分析:(6) bio后台服务
redis的BIO（Background I/O）顾名思义：后台I/O服务，是Redis中用于处理一些需要异步执行的后台任务的一个线程池。这些后台任务包括了文件同步（fsync）、文件关闭（close）、对象释放（free）等，以避免这些操作阻塞主线程，从而影响处理效率。
由此我们也可以看出，redis其实并非真正的单线程程序，只是它的主要数据库读写处理流程是在主线程完成的，但内部是会启动多种线程来辅助提升效率的。除了BIO的线程外，后续我还会讲解网络IO多线程的代码实现，也会涉及到整体主流程框架的实现原理。
BIO是基于生产者消费者模型实现的，使用锁和条件变量来同步。redis为每种BIO操作类型创建了一个后台线程，每个线程处理一个特定类型的任务队列。当任务到来时，主线程将任务添加到相应操作类型的任务队列中，并唤醒等待在该队列上的线程。后台线程循环检查并取出队列中的任务执行，直到队列为空。
下面看一下主要的代码实现
BIO主要结构定义：
// 线程对象数组
static pthread_t bio_threads[BIO_NUM_OPS];
// 线程锁数组
static pthread_mutex_t bio_mutex[BIO_NUM_OPS];
// 任务通知条件等待对象数组
static pthread_cond_t bio_newjob_cond[BIO_NUM_OPS];
// 任务执行完毕条件等待对象数组
static pthread_cond_t bio_step_cond[BIO_NUM_OPS];
// 任务链表数组
static list *bio_jobs[BIO_NUM_OPS];
/* 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. */
// 任务等待数数组
static unsigned long long bio_pending[BIO_NUM_OPS];
/* This structure represents a background Job. It is only used locally to this
* file as the API does not expose the internals at all. */
typedef union bio_job {
/* Job specific arguments.*/
struct {
// 文件句柄
int fd; /* Fd for file based background jobs */
// 是否在关闭前刷新到磁盘(文件系统缓冲区)
unsigned need_fsync:1; /* A flag to indicate that a fsync is required before
* the file is closed. */
} fd_args;
struct {
// 延迟释放函数
lazy_free_fn *free_fn; /* Function that will free the provided arguments */
// 释放参数
void *free_args[]; /* List of arguments to be passed to the free function */
} free_args;
} bio_job;
BIO初始化：
void bioInit(void) {
pthread_attr_t attr;
pthread_t thread;
size_t stacksize;
int j;
/* Initialization of state vars and objects */
// 初始化所有线程中需要用到的数组
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 */
// 设置线程栈大小
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. */
// 创建所有后台任务线程
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;
}
}
BIO任务提交：
void bioSubmitJob(int type, bio_job *job) {
// 根据type添加任务到相应链表中,然后通知处理线程处理
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]);
}
void bioCreateLazyFreeJob(lazy_free_fn free_fn, int arg_count, ...) {
va_list valist;
/* Allocate memory for the job structure and all required
* arguments */
// 分配函数指针以及参数所需要的空间
bio_job *job = zmalloc(sizeof(*job) + sizeof(void *) * (arg_count));
job->free_args.free_fn = free_fn;
// 存储所有参数
va_start(valist, arg_count);
for (int i = 0; i < arg_count; i++) {
job->free_args.free_args[i] = va_arg(valist, void *);
}
va_end(valist);
// 提交延迟释放任务
bioSubmitJob(BIO_LAZY_FREE, job);
}
void bioCreateCloseJob(int fd, int need_fsync) {
bio_job *job = zmalloc(sizeof(*job));
job->fd_args.fd = fd;
job->fd_args.need_fsync = need_fsync;
// 提交关闭文件任务
bioSubmitJob(BIO_CLOSE_FILE, job);
}
void bioCreateFsyncJob(int fd) {
bio_job *job = zmalloc(sizeof(*job));
job->fd_args.fd = fd;
// 提交文件刷新到磁盘任务
bioSubmitJob(BIO_AOF_FSYNC, job);
}
BIO任务线程函数：
void *bioProcessBackgroundJobs(void *arg) {
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;
}
// 设置线程名
switch (type) {
case BIO_CLOSE_FILE:
redis_set_thread_title("bio_close_file");
break;
case BIO_AOF_FSYNC:
redis_set_thread_title("bio_aof_fsync");
break;
case BIO_LAZY_FREE:
redis_set_thread_title("bio_lazy_free");
break;
}
// 设置cpu和线程的绑定
redisSetCpuAffinity(server.bio_cpulist);
// 设置线程运行可以随时被取消
makeThreadKillable();
pthread_mutex_lock(&bio_mutex[type]);
/* Block SIGALRM so we are sure that only the main thread will
* receive the watchdog signal. */
// 屏蔽SIGALRM
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. */
// 任务链表为空时进行等待
if (listLength(bio_jobs[type]) == 0) {
pthread_cond_wait(&bio_newjob_cond[type],&bio_mutex[type]);
continue;
}
/* Pop the job from the queue. */
// 取出任务
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) {
if (job->fd_args.need_fsync) {
// 刷新到磁盘
redis_fsync(job->fd_args.fd);
}
// 关闭文件
close(job->fd_args.fd);
} else if (type == BIO_AOF_FSYNC) {
/* The fd may be closed by main thread and reused for another
* socket, pipe, or file. We just ignore these errno because
* aof fsync did not really fail. */
// 刷新到磁盘
if (redis_fsync(job->fd_args.fd) == -1 &&
errno != EBADF && errno != EINVAL)
{
int last_status;
atomicGet(server.aof_bio_fsync_status,last_status);
atomicSet(server.aof_bio_fsync_status,C_ERR);
atomicSet(server.aof_bio_fsync_errno,errno);
if (last_status == C_OK) {
serverLog(LL_WARNING,
"Fail to fsync the AOF file: %s",strerror(errno));
}
} else {
atomicSet(server.aof_bio_fsync_status,C_OK);
}
} else if (type == BIO_LAZY_FREE) {
// 执行释放函数
job->free_args.free_fn(job->free_args.free_args);
} 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. */
// 通知其它执行 bioWaitStepOfType() 的线程结束等待
pthread_cond_broadcast(&bio_step_cond[type]);
}
}
/* 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]);
// 返回任务数量
val = bio_pending[type];
pthread_mutex_unlock(&bio_mutex[type]);
return val;
}
unsigned long long bioWaitStepOfType(int type) {
unsigned long long val;
pthread_mutex_lock(&bio_mutex[type]);
val = bio_pending[type];
if (val != 0) {
// 等待任务完成通知
pthread_cond_wait(&bio_step_cond[type],&bio_mutex[type]);
val = bio_pending[type];
}
pthread_mutex_unlock(&bio_mutex[type]);
return val;
}
未经许可，请勿转载！作者：jwybobo2007
#redis#
