redis源码笔记 - bio

作者在bio.c的头注释中对设计进行了详细的介绍
/* Background I/O service for Redis.
   这个文件是redis后台IO服务的实现
 *
 * 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.
   这个文件负责我们需要在后台执行的操作。现在redis的版本中只有一类的操作，后台的close 系统调用。
   为了避免一个文件最后的owner在执行close操作带来的unlink使得阻塞server，将这类操作用单独的后台线程来执行
 *
 * 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.
   每种作业类型一个queue。每个线程在它的queue里等待新的job到来。并且按照FIFO的顺序处理作业。
 *
 * 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.
   作业完成后，其creator无法得到通知。
 */

现在的两类作业类型：1.close 2.aof_fsync

/* Background job opcodes */
#define REDIS_BIO_CLOSE_FILE    0 /* Deferred close(2) syscall. */
#define REDIS_BIO_AOF_FSYNC     1 /* Deferred AOF fsync. */

  1 #include "redis.h"
  2 #include "bio.h"
  3 
    //使用互斥量+条件变量，作为线程的保护条件
  4 static pthread_mutex_t bio_mutex[REDIS_BIO_NUM_OPS];
  5 static pthread_cond_t bio_condvar[REDIS_BIO_NUM_OPS];
    //两类作业的队列
  6 static list *bio_jobs[REDIS_BIO_NUM_OPS];
  7 /* The following array is used to hold the number of pending jobs for every
  8  * OP type. This allows us to export the bioPendingJobsOfType() API that is
  9  * useful when the main thread wants to perform some operation that may involve
 10  * objects shared with the background thread. The main thread will just wait
 11  * that there are no longer jobs of this type to be executed before performing
 12  * the sensible operation. This data is also useful for reporting. */
 13 static unsigned long long bio_pending[REDIS_BIO_NUM_OPS];
 14 
 15 /* This structure represents a background Job. It is only used locally to this
 16  * file as the API deos not expose the internals at all. */
 17 struct bio_job {
 18     time_t time; /* Time at which the job was created. */
 19     /* Job specific arguments pointers. If we need to pass more than three
 20      * arguments we can just pass a pointer to a structure or alike. */
 21     void *arg1, *arg2, *arg3;
 22 };
 23 
 24 void *bioProcessBackgroundJobs(void *arg);
 25 
 26 /* Make sure we have enough stack to perform all the things we do in the
 27  * main thread. */
 28 #define REDIS_THREAD_STACK_SIZE (1024*1024*4)
 29 
 30 /* Initialize the background system, spawning the thread. */
 31 void bioInit(void) {
 32     pthread_attr_t attr;
 33     pthread_t thread;
 34     size_t stacksize;
 35     int j;
 36 
 37     /* Initialization of state vars and objects */
 38     for (j = 0; j < REDIS_BIO_NUM_OPS; j++) {
 39         pthread_mutex_init(&bio_mutex[j],NULL);
 40         pthread_cond_init(&bio_condvar[j],NULL);
 41         bio_jobs[j] = listCreate();
 42         bio_pending[j] = 0;
 43     }
 44 
 45     /* Set the stack size as by default it may be small in some system */
 46     pthread_attr_init(&attr);
 47     pthread_attr_getstacksize(&attr,&stacksize);
 48     if (!stacksize) stacksize = 1; /* The world is full of Solaris Fixes */
 49     while (stacksize < REDIS_THREAD_STACK_SIZE) stacksize *= 2;
 50     pthread_attr_setstacksize(&attr, stacksize);
 51 
 52     /* Ready to spawn our threads. We use the single argument the thread
 53      * function accepts in order to pass the job ID the thread is
 54      * responsible of. */
 55     for (j = 0; j < REDIS_BIO_NUM_OPS; j++) {
 56         void *arg = (void*)(unsigned long) j;
 57         if (pthread_create(&thread,&attr,bioProcessBackgroundJobs,arg) != 0) {
 58             redisLog(REDIS_WARNING,"Fatal: Can't initialize Background Jobs.");
 59             exit(1);
 60         }
 61     }
 62 }
 63 
 64 void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3) {
 65     struct bio_job *job = zmalloc(sizeof(*job));
 66 
 67     job->time = time(NULL);
 68     job->arg1 = arg1;
 69     job->arg2 = arg2;
 70     job->arg3 = arg3;
 71     pthread_mutex_lock(&bio_mutex[type]);
 72     listAddNodeTail(bio_jobs[type],job);
 73     bio_pending[type]++;
 74     pthread_cond_signal(&bio_condvar[type]);
 75     pthread_mutex_unlock(&bio_mutex[type]);
 76 }
 77 
 78 void *bioProcessBackgroundJobs(void *arg) {
 79     struct bio_job *job;
 80     unsigned long type = (unsigned long) arg;
 81 
 82     pthread_detach(pthread_self());
 83     pthread_mutex_lock(&bio_mutex[type]);
 84     while(1) {
 85         listNode *ln;
 86 
 87         /* The loop always starts with the lock hold. */
 88         if (listLength(bio_jobs[type]) == 0) {
 89             pthread_cond_wait(&bio_condvar[type],&bio_mutex[type]);
 90             continue;
 91         }
 92         /* Pop the job from the queue. */
 93         ln = listFirst(bio_jobs[type]);
 94         job = ln->value;
 95         /* It is now possible to unlock the background system as we know have
 96          * a stand alone job structure to process.*/
 97         pthread_mutex_unlock(&bio_mutex[type]);
 98 
 99         /* Process the job accordingly to its type. */
100         if (type == REDIS_BIO_CLOSE_FILE) {
101             close((long)job->arg1);
102         } else if (type == REDIS_BIO_AOF_FSYNC) {
103             aof_fsync((long)job->arg1);
104         } else {
105             redisPanic("Wrong job type in bioProcessBackgroundJobs().");
106         }
107         zfree(job);
108 
109         /* Lock again before reiterating the loop, if there are no longer
110          * jobs to process we'll block again in pthread_cond_wait(). */
111         pthread_mutex_lock(&bio_mutex[type]);
112         listDelNode(bio_jobs[type],ln);
113         bio_pending[type]--;
114     }
115 }
116 
117 /* Return the number of pending jobs of the specified type. */
118 unsigned long long bioPendingJobsOfType(int type) {
119     unsigned long long val;
120     pthread_mutex_lock(&bio_mutex[type]);
121     val = bio_pending[type];
122     pthread_mutex_unlock(&bio_mutex[type]);
123     return val;
124 }