MySQL数据库线程缓冲池详解
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=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：
staticboolcache_thread(){safe_mutex_assert_owner(&LOCK_thread_count);if(cached_thread_count<thread_cache_size&&!abort_loop&&!kill_cached_threads){/*Don'tkillthethread,justputitincacheforreuse*/DBUG_PRINT("info",("Addingthreadtocache"));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;//Forstore_globals(void)thd->store_globals();/*THD::mysys_var::abortisassociatedwithphysicalthreadratherthanwithTHDobject.SoweneedtoresetthisflagbeforeusingthisthreadforhandlingofnewTHDobject/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：
intpthread_cond_timedwait(pthread_cond_t*cond,pthread_mutex_t*mutex,structtimespec*abstime)intresult;longtimeout;unionft64now;if(abstime!=NULL){GetSystemTimeAsFileTime(&now.ft);/*Calculatetimelefttoabstime-subtractstarttimefromcurrenttime(valuesarein100nsunits)-converttomillisecbydividingwith10000*/timeout=(long)((abstime->tv.i64-now.i64)/10000);/*Don'tallowthetimeouttobenegative*/if(timeout<0)timeout=0L;/*Makesurethecalucatedtimeoutdoesnotexceedoriginaltimeoutvaluewhichcouldcause"waitforever"ifsystemtimechanges*/if(timeout>abstime->max_timeout_msec)timeout=abstime->max_timeout_msec;}else{/*Notimespecified;don'texpire*/timeout=INFINITE;}/*Blockaccessifpreviousbroadcasthasn'tfinished.Thisisjustforsafetyandshouldnormallynotaffectthetotaltimespentinthisfunction.*/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'rethelastwaitertobenotifiedortostopwaiting,soresetthemanualevent.*//*Closebroadcastgate*/ResetEvent(cond->events[BROADCAST]);/*Openblockgate*/SetEvent(cond->broadcast_block_event);}LeaveCriticalSection(&cond->lock_waiting);EnterCriticalSection(mutex);returnresult==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：
voidcreate_thread_to_handle_connection(THD*thd){if(cached_thread_count>wake_thread){/*Getthreadfromcache*/thread_cache.append(thd);wake_thread++;pthread_cond_signal(&COND_thread_cache);}Else...}
关于MySQL数据库线程缓冲池的相关知识就介绍到这里了O(∩_∩)O~
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