/* 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 < 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;
}

/* 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 // 线程总数，redis主线程+3个后台线程