知识大全 MySQL数据库线程缓冲池详解

Posted 2022-07-15 知

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　　MySQL数据库线程缓冲池的相关知识是本文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=

　　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

以下是代码片段 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= ; thd >thr_create_utime= my_micro_time(); threads append(thd); return( ); return( );

　　上面我们的启动参数设置线程缓冲区为此时对应代码里面的thread_cache_size = 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

以下是代码片段 int pthread_cond_timedwait(pthread_cond_t *cond pthread_mutex_t *mutex struct timespec *abstime) int result; long timeout; union ft now; if( abstime != NULL ) GetSystemTimeAsFileTime(&now ft); /* Calculate time left to abstime subtract start time from current time(values are in ns units) convert to millisec by dividing with */ timeout= (long)((abstime >tv i now i ) / ); /* Don t allow the timeout to be negative */ if (timeout < ) timeout= L; /* 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( cond >events FALSE timeout); EnterCriticalSection(&cond >lock_waiting); cond >waiting ; if (cond >waiting == ) /* 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 : ;

　　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

以下是代码片段 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 cha138/Article/program/MySQL/201311/29579