MySQL数据库线程缓冲池的相关知识是该篇文章我们主要要介绍的内容，MySQL数据库支持线程缓存，在多线程连接模式下，如果连接断开后，将这个线程放入空闲线程缓冲区，在下次有连接到来时，先去缓冲池中查找是否有空闲线程，有则用之，无则创建。启动时可以设置线程缓冲池的数目：Mysqld.exe --thread_cache_size=10。
在一个连接断开时，会调用cache_thread函数，将空闲的线程加入到cache中，以备后用。源码如下：
static bool cache_thread() { safe_mutex_assert_owner(&LOCK_thread_count); if ( cached_thread_count < thread_cache_size && ! abort_loop && !kill_cached_threads) { /* Don't kill the thread, just put it in cache for reuse */ DBUG_PRINT("info", ("Adding thread to cache")); cached_thread_count++; while (!abort_loop && ! wake_thread && ! kill_cached_threads) (void) pthread_cond_wait(&COND_thread_cache, &LOCK_thread_count); cached_thread_count--; if (kill_cached_threads) pthread_cond_signal(&COND_flush_thread_cache); if (wake_thread) { THD *thd; wake_thread--; thd= thread_cache.get(); thd->thread_stack= (char*) &thd; // For store_globals (void) thd->store_globals(); /* THD::mysys_var::abort is associated with physical thread rather than with THD object. So we need to reset this flag before using this thread for handling of new THD object/connection. */ thd->mysys_var->abort= 0; thd->thr_create_utime= my_micro_time(); threads.append(thd); return(1); } } return(0); }
上面我们的启动参数设置线程缓冲区为10，此时对应代码里面的thread_cache_size = 10，cached_thread_count记录
了此刻cache中的空闲线程数目，只有在cache未满的情况下，才会将新的空闲线程加入缓冲池中。加入到缓冲区其实就是将线
程挂起，pthread_cond_wait函数便是线程等待函数，在此函数中，会调用WaitForMultipleObjects进行事件等待。具体源码
如下：
int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex, struct timespec *abstime) int result; long timeout; union ft64 now; if( abstime != NULL ) { GetSystemTimeAsFileTime(&now.ft); /* Calculate time left to abstime - subtract start time from current time(values are in 100ns units) - convert to millisec by dividing with 10000 */ timeout= (long)((abstime->tv.i64 - now.i64) / 10000); /* Don't allow the timeout to be negative */ if (timeout < 0) timeout= 0L; /* Make sure the calucated timeout does not exceed original timeout value which could cause "wait for ever" if system time changes */ if (timeout > abstime->max_timeout_msec) timeout= abstime->max_timeout_msec; } else { /* No time specified; don't expire */ timeout= INFINITE; } /* Block access if previous broadcast hasn't finished. This is just for safety and should normally not affect the total time spent in this function. */ WaitForSingleObject(cond->broadcast_block_event, INFINITE); EnterCriticalSection(&cond->lock_waiting); cond->waiting++; LeaveCriticalSection(&cond->lock_waiting); LeaveCriticalSection(mutex); result= WaitForMultipleObjects(2, cond->events, FALSE, timeout); EnterCriticalSection(&cond->lock_waiting); cond->waiting--; if (cond->waiting == 0) { /* We're the last waiter to be notified or to stop waiting, so reset the manual event. */ /* Close broadcast gate */ ResetEvent(cond->events[BROADCAST]); /* Open block gate */ SetEvent(cond->broadcast_block_event); } LeaveCriticalSection(&cond->lock_waiting); EnterCriticalSection(mutex); return result == WAIT_TIMEOUT ETIMEDOUT : 0; }
此处是等待时间，何处进行事件通知呢？我们再次来到上篇所提及的为新的连接创建线程的代码中：
void create_thread_to_handle_connection(THD *thd) { if (cached_thread_count > wake_thread) { /* Get thread from cache */ thread_cache.append(thd); wake_thread++; pthread_cond_signal(&COND_thread_cache); } Else ... }
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