The query which can be used to identify Top Wait Stats is given below. It is used to Isolate top waits for server instance since last restart.
This query has been really helpfull in identifying various issues as I/O issues, CPU Bottlenecks etc.
Top Waits Stats rollup query
-- Isolate top waits for server instance since last restart
WITH Waits AS
(SELECT wait_type, wait_time_ms / 1000. AS wait_time_s,
100. * wait_time_ms / SUM(wait_time_ms) OVER() AS pct,
ROW_NUMBER() OVER(ORDER BY wait_time_ms DESC) AS rn
WHERE wait_type NOT IN ('CLR_SEMAPHORE','LAZYWRITER_SLEEP','RESOURCE_QUEUE','SLEEP_TASK'
CAST(W1.wait_time_s AS DECIMAL(12, 2)) AS wait_time_s,
CAST(W1.pct AS DECIMAL(12, 2)) AS pct,
CAST(SUM(W2.pct) AS DECIMAL(12, 2)) AS running_pct
FROM Waits AS W1
INNER JOIN Waits AS W2
ON W2.rn <= W1.rn GROUP BY W1.rn, W1.wait_type, W1.wait_time_s, W1.pct HAVING SUM(W2.pct) - W1.pct < 95; -- percentage threshold -- This gives you an idea of the most significant wait type for the instance since -- SQL Server was started or the last time the wait stats were cleared by -- running the DBCC SQLPERF('sys.dm_os_wait_stats', CLEAR); command -- If your server is running well, you should not "fixate" as much on the top wait type... -- Common Significant Wait types with BOL explanations -- *** Network Related Waits *** -- ASYNC_NETWORK_IO Occurs on network writes when the task is blocked behind the network -- *** Locking Waits *** -- LCK_M_IX Occurs when a task is waiting to acquire an Intent Exclusive (IX) lock -- LCK_M_IU Occurs when a task is waiting to acquire an Intent Update (IU) lock -- LCK_M_S Occurs when a task is waiting to acquire a Shared lock -- *** I/O Related Waits *** -- ASYNC_IO_COMPLETION Occurs when a task is waiting for I/Os to finish -- IO_COMPLETION Occurs while waiting for I/O operations to complete. -- This wait type generally represents non-data page I/Os. Data page I/O completion waits appear -- as PAGEIOLATCH_* waits -- PAGEIOLATCH_SH Occurs when a task is waiting on a latch for a buffer that is in an I/O request. -- The latch request is in Shared mode. Long waits may indicate problems with the disk subsystem. -- PAGEIOLATCH_EX Occurs when a task is waiting on a latch for a buffer that is in an I/O request. -- The latch request is in Exclusive mode. Long waits may indicate problems with the disk subsystem. -- WRITELOG Occurs while waiting for a log flush to complete. -- Common operations that cause log flushes are checkpoints and transaction commits. -- PAGELATCH_EX Occurs when a task is waiting on a latch for a buffer that is not in an I/O request. -- The latch request is in Exclusive mode. -- BACKUPIO Occurs when a backup task is waiting for data, or is waiting for a buffer in which to store data -- *** CPU Related Waits *** -- SOS_SCHEDULER_YIELD Occurs when a task voluntarily yields the scheduler for other tasks to execute. -- During this wait the task is waiting for its quantum to be renewed. -- THREADPOOL Occurs when a task is waiting for a worker to run on. -- This can indicate that the maximum worker setting is too low, or that batch executions are taking -- unusually long, thus reducing the number of workers available to satisfy other batches. -- CX_PACKET Occurs when trying to synchronize the query processor exchange iterator -- You may consider lowering the degree of parallelism if contention on this wait type becomes a problem -- Often caused by missing indexes or poorly written queries