告訴我, 究竟我的SQL Server慢在哪里?

你可以使用下面的語句來使用sys.dm_os_wait_stats這個DMV得到線程的等待信息(線程在等什么, 等了多久)的統計數值.

WITH [Waits] AS
    (SELECT
        [wait_type],
        [wait_time_ms] / 1000.0 AS [WaitS],
        ([wait_time_ms] - [signal_wait_time_ms]) / 1000.0 AS [ResourceS],
        [signal_wait_time_ms] / 1000.0 AS [SignalS],
        [waiting_tasks_count] AS [WaitCount],
        100.0 * [wait_time_ms] / SUM ([wait_time_ms]) OVER() AS [Percentage],
        ROW_NUMBER() OVER(ORDER BY [wait_time_ms] DESC) AS [RowNum]
    FROM sys.dm_os_wait_stats
    WHERE [wait_type] NOT IN (
        N'BROKER_EVENTHANDLER',             N'BROKER_RECEIVE_WAITFOR',
        N'BROKER_TASK_STOP',                N'BROKER_TO_FLUSH',
        N'BROKER_TRANSMITTER',              N'CHECKPOINT_QUEUE',
        N'CHKPT',                           N'CLR_AUTO_EVENT',
        N'CLR_MANUAL_EVENT',                N'CLR_SEMAPHORE',
        N'DBMIRROR_DBM_EVENT',              N'DBMIRROR_EVENTS_QUEUE',
        N'DBMIRROR_WORKER_QUEUE',           N'DBMIRRORING_CMD',
        N'DIRTY_PAGE_POLL',                 N'DISPATCHER_QUEUE_SEMAPHORE',
        N'EXECSYNC',                        N'FSAGENT',
        N'FT_IFTS_SCHEDULER_IDLE_WAIT',     N'FT_IFTSHC_MUTEX',
        N'HADR_CLUSAPI_CALL',               N'HADR_FILESTREAM_IOMGR_IOCOMPLETION',
        N'HADR_LOGCAPTURE_WAIT',            N'HADR_NOTIFICATION_DEQUEUE',
        N'HADR_TIMER_TASK',                 N'HADR_WORK_QUEUE',
        N'KSOURCE_WAKEUP',                  N'LAZYWRITER_SLEEP',
        N'LOGMGR_QUEUE',                    N'ONDEMAND_TASK_QUEUE',
        N'PWAIT_ALL_COMPONENTS_INITIALIZED',
        N'QDS_PERSIST_TASK_MAIN_LOOP_SLEEP',
        N'QDS_CLEANUP_STALE_QUERIES_TASK_MAIN_LOOP_SLEEP',
        N'REQUEST_FOR_DEADLOCK_SEARCH',     N'RESOURCE_QUEUE',
        N'SERVER_IDLE_CHECK',               N'SLEEP_BPOOL_FLUSH',
        N'SLEEP_DBSTARTUP',                 N'SLEEP_DCOMSTARTUP',
        N'SLEEP_MASTERDBREADY',             N'SLEEP_MASTERMDREADY',
        N'SLEEP_MASTERUPGRADED',            N'SLEEP_MSDBSTARTUP',
        N'SLEEP_SYSTEMTASK',                N'SLEEP_TASK',
        N'SLEEP_TEMPDBSTARTUP',             N'SNI_HTTP_ACCEPT',
        N'SP_SERVER_DIAGNOSTICS_SLEEP',     N'SQLTRACE_BUFFER_FLUSH',
        N'SQLTRACE_INCREMENTAL_FLUSH_SLEEP',
        N'SQLTRACE_WAIT_ENTRIES',           N'WAIT_FOR_RESULTS',
        N'WAITFOR',                         N'WAITFOR_TASKSHUTDOWN',
        N'WAIT_XTP_HOST_WAIT',              N'WAIT_XTP_OFFLINE_CKPT_NEW_LOG',
        N'WAIT_XTP_CKPT_CLOSE',             N'XE_DISPATCHER_JOIN',
        N'XE_DISPATCHER_WAIT',              N'XE_TIMER_EVENT')
    AND [waiting_tasks_count] > 0
 )
SELECT
    MAX ([W1].[wait_type]) AS [WaitType],
    CAST (MAX ([W1].[WaitS]) AS DECIMAL (16,2)) AS [Wait_S],
    CAST (MAX ([W1].[ResourceS]) AS DECIMAL (16,2)) AS [Resource_S],
    CAST (MAX ([W1].[SignalS]) AS DECIMAL (16,2)) AS [Signal_S],
    MAX ([W1].[WaitCount]) AS [WaitCount],
    CAST (MAX ([W1].[Percentage]) AS DECIMAL (5,2)) AS [Percentage],
    CAST ((MAX ([W1].[WaitS]) / MAX ([W1].[WaitCount])) AS DECIMAL (16,4)) AS [AvgWait_S],
    CAST ((MAX ([W1].[ResourceS]) / MAX ([W1].[WaitCount])) AS DECIMAL (16,4)) AS [AvgRes_S],
    CAST ((MAX ([W1].[SignalS]) / MAX ([W1].[WaitCount])) AS DECIMAL (16,4)) AS [AvgSig_S]
FROM [Waits] AS [W1]
INNER JOIN [Waits] AS [W2]
    ON [W2].[RowNum] <= [W1].[RowNum]
GROUP BY [W1].[RowNum]
HAVING SUM ([W2].[Percentage]) - MAX ([W1].[Percentage]) < 95; -- percentage threshold
GO

一些說明:

SQL Server會永久地跟蹤為什么執行的線程會處於等待狀態. 你可以從SQL Server中得到這些信息, 然后縮小性能問題的調查范圍.

有些人一開始調查性能問題就會看幾次當下的線程在等待着什么, 然后嘗試去弄清楚為什么. 而事實上是, '等待'這回事兒是always發生的, SQL的scheduleing系統就是這么工作的. 下面讓我們來看一下scheduleing大致的工作流程吧.

正在使用CPU的線程(處於RUNNING狀態)會在它需要某種資源的時候停下來. 該線程會被移到一個叫做SUSPENDED的線程的無序列表中. 同時, 在RUNNABLE隊列(先進先出)中等待CPU的下一個線程會變為RUNNING狀態. 如果在SUSPENDED列表中的一個線程得到通知說它想要的資源可用了, 它就會轉到RUNNABLE隊列中的底部. 就這樣, 線程們像鍾表一樣的從RUNNING到SUSPENDED, 在到RUNNABLE, 再到RUNNING的循環, 直到任務完成.

SQL Server跟蹤從離開RUNNING狀態到再回到RUNNING狀態的時間(叫做wait time), 和花在RUNNABLE隊列中的時間(叫做signal wait time). 我們需要得到在SUSPENDED隊列中的時間, 方法就是從整個的wait time中減掉signal wait time.

 

下面節選了一些常見的等待類型, 並作出解釋. 更多等待類型的信息請看原文.

• PAGEIOLATCH_XX: 線程正在等待一個數據page從磁盤中讀到buffer pool中.
• LCK_M_X: 線程正在等待被在某資源上賦予一個排他鎖.
• CXPACKET: 並行線程中某一個線程在執行時, 整個query都會被block, 這時該數值會被累加. 具體見原文.
• WRITELOG: 日志管理系統在等待log被flush到磁盤上.

 

信息來源

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Wait statistics, or please tell me where it hurts

http://www.sqlskills.com/blogs/paul/wait-statistics-or-please-tell-me-where-it-hurts/