MySQL數據庫線程緩沖池詳解

MySQL數據庫線程緩沖池的相關知識是本文我們主要要介紹的內容，MySQL數據庫支持線程緩存，在多線程連接模式下，如果連接斷開後，將這個線 程放入空閒線程緩沖區，在下次有連接到來時，先去緩沖池中查找是否有空閒線程，有則用之，無則創建。啟動時可以設置線程緩沖池的數 目：Mysqld.exe --thread_cache_size=10。

　　在一個連接斷開時，會調用cache_thread函數，將空閒的線程加入到cache中，以備後用。源碼如下：

 

以下是代碼片段：

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= 0;
thd->thr_create_utime= my_micro_time();
threads.append(thd);
return(1);
}
}
return(0);
}

　　上面我們的啟動參數設置線程緩沖區為10，此時對應代碼裡面的thread_cache_size = 10，cached_thread_count記錄

　　了此刻cache中的空閒線程數目，只有在cache未滿的情況下，才會將新的空閒線程加入緩沖池中。加入到緩沖區其實就是將線

 

　　程掛起，pthread_cond_wait函數便是線程等待函數，在此函數中，會調用WaitForMultipleObjects進行事件等待。具體源碼

　　如下：

 

以下是代碼片段：

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;
}

　　此處是等待時間，何處進行事件通知呢？我們再次來到上篇所提及的為新的連接創建線程的代碼中：

 

以下是代碼片段：

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
...
}  

　　關於MySQL數據庫線程緩沖池的相關知識就介紹到這裡了，希望本次的介紹能夠對您有所收獲!