As a human being, your SQL Server tries to express its pain point with you — but not in words. It whispers through wait stats, sending subtle signals when something is off. But, you have to learn how to decode these signals.
In this article, we will decode the most common SQL Server wait types—like CXPACKET, PAGEIOLATCH, and SOS_SCHEDULER_YIELD—and understand what they are quietly telling you about your server’s workload, health, and bottlenecks. If you have ever wondered why your queries slow down despite healthy hardware, the answer might just be in these whispers.
Wait Statistics
When you submit a SELECT query to SQL Server, the ideal scenario is that the CPU quickly processes and returns the result. However in practical, many requests compete for limited server resources. This can lead to delays — some requests have to wait before they can move forward. For example, if one query is modifying a table row then another select query might have to wait until it is free. SQL Server keeps track of these waits. It notes the duration of the wait,
and the cause of the wait (wait type). These records are called Wait Statistics/Wait Stats.
You can find out the cause of the waits and tune your SQL Server by examining two Dynamic Management Views:
- sys.dm_os_wait_stats (or sys.dm_db_wait_stats on Azure SQL Database) – cumulative duration of all wait types
- sys.dm_os_waiting_tasks – wait statistics of currently running queries
Finding Out Top Wait Types
Paul Randal compiled all common wait types and wrote below scripts for identifying top wait types of your workload.
-- Last updated October 1, 2021
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 (
-- These wait types are almost 100% never a problem and so they are
-- filtered out to avoid them skewing the results. Click on the URL
-- for more information.
N'BROKER_EVENTHANDLER', -- https://www.sqlskills.com/help/waits/BROKER_EVENTHANDLER
N'BROKER_RECEIVE_WAITFOR', -- https://www.sqlskills.com/help/waits/BROKER_RECEIVE_WAITFOR
N'BROKER_TASK_STOP', -- https://www.sqlskills.com/help/waits/BROKER_TASK_STOP
N'BROKER_TO_FLUSH', -- https://www.sqlskills.com/help/waits/BROKER_TO_FLUSH
N'BROKER_TRANSMITTER', -- https://www.sqlskills.com/help/waits/BROKER_TRANSMITTER
N'CHECKPOINT_QUEUE', -- https://www.sqlskills.com/help/waits/CHECKPOINT_QUEUE
N'CHKPT', -- https://www.sqlskills.com/help/waits/CHKPT
N'CLR_AUTO_EVENT', -- https://www.sqlskills.com/help/waits/CLR_AUTO_EVENT
N'CLR_MANUAL_EVENT', -- https://www.sqlskills.com/help/waits/CLR_MANUAL_EVENT
N'CLR_SEMAPHORE', -- https://www.sqlskills.com/help/waits/CLR_SEMAPHORE
-- Maybe comment this out if you have parallelism issues
N'CXCONSUMER', -- https://www.sqlskills.com/help/waits/CXCONSUMER
-- Maybe comment these four out if you have mirroring issues
N'DBMIRROR_DBM_EVENT', -- https://www.sqlskills.com/help/waits/DBMIRROR_DBM_EVENT
N'DBMIRROR_EVENTS_QUEUE', -- https://www.sqlskills.com/help/waits/DBMIRROR_EVENTS_QUEUE
N'DBMIRROR_WORKER_QUEUE', -- https://www.sqlskills.com/help/waits/DBMIRROR_WORKER_QUEUE
N'DBMIRRORING_CMD', -- https://www.sqlskills.com/help/waits/DBMIRRORING_CMD
N'DIRTY_PAGE_POLL', -- https://www.sqlskills.com/help/waits/DIRTY_PAGE_POLL
N'DISPATCHER_QUEUE_SEMAPHORE', -- https://www.sqlskills.com/help/waits/DISPATCHER_QUEUE_SEMAPHORE
N'EXECSYNC', -- https://www.sqlskills.com/help/waits/EXECSYNC
N'FSAGENT', -- https://www.sqlskills.com/help/waits/FSAGENT
N'FT_IFTS_SCHEDULER_IDLE_WAIT', -- https://www.sqlskills.com/help/waits/FT_IFTS_SCHEDULER_IDLE_WAIT
N'FT_IFTSHC_MUTEX', -- https://www.sqlskills.com/help/waits/FT_IFTSHC_MUTEX
-- Maybe comment these six out if you have AG issues
N'HADR_CLUSAPI_CALL', -- https://www.sqlskills.com/help/waits/HADR_CLUSAPI_CALL
N'HADR_FILESTREAM_IOMGR_IOCOMPLETION', -- https://www.sqlskills.com/help/waits/HADR_FILESTREAM_IOMGR_IOCOMPLETION
N'HADR_LOGCAPTURE_WAIT', -- https://www.sqlskills.com/help/waits/HADR_LOGCAPTURE_WAIT
N'HADR_NOTIFICATION_DEQUEUE', -- https://www.sqlskills.com/help/waits/HADR_NOTIFICATION_DEQUEUE
N'HADR_TIMER_TASK', -- https://www.sqlskills.com/help/waits/HADR_TIMER_TASK
N'HADR_WORK_QUEUE', -- https://www.sqlskills.com/help/waits/HADR_WORK_QUEUE
N'KSOURCE_WAKEUP', -- https://www.sqlskills.com/help/waits/KSOURCE_WAKEUP
N'LAZYWRITER_SLEEP', -- https://www.sqlskills.com/help/waits/LAZYWRITER_SLEEP
N'LOGMGR_QUEUE', -- https://www.sqlskills.com/help/waits/LOGMGR_QUEUE
N'MEMORY_ALLOCATION_EXT', -- https://www.sqlskills.com/help/waits/MEMORY_ALLOCATION_EXT
N'ONDEMAND_TASK_QUEUE', -- https://www.sqlskills.com/help/waits/ONDEMAND_TASK_QUEUE
N'PARALLEL_REDO_DRAIN_WORKER', -- https://www.sqlskills.com/help/waits/PARALLEL_REDO_DRAIN_WORKER
N'PARALLEL_REDO_LOG_CACHE', -- https://www.sqlskills.com/help/waits/PARALLEL_REDO_LOG_CACHE
N'PARALLEL_REDO_TRAN_LIST', -- https://www.sqlskills.com/help/waits/PARALLEL_REDO_TRAN_LIST
N'PARALLEL_REDO_WORKER_SYNC', -- https://www.sqlskills.com/help/waits/PARALLEL_REDO_WORKER_SYNC
N'PARALLEL_REDO_WORKER_WAIT_WORK', -- https://www.sqlskills.com/help/waits/PARALLEL_REDO_WORKER_WAIT_WORK
N'PREEMPTIVE_OS_FLUSHFILEBUFFERS', -- https://www.sqlskills.com/help/waits/PREEMPTIVE_OS_FLUSHFILEBUFFERS
N'PREEMPTIVE_XE_GETTARGETSTATE', -- https://www.sqlskills.com/help/waits/PREEMPTIVE_XE_GETTARGETSTATE
N'PVS_PREALLOCATE', -- https://www.sqlskills.com/help/waits/PVS_PREALLOCATE
N'PWAIT_ALL_COMPONENTS_INITIALIZED', -- https://www.sqlskills.com/help/waits/PWAIT_ALL_COMPONENTS_INITIALIZED
N'PWAIT_DIRECTLOGCONSUMER_GETNEXT', -- https://www.sqlskills.com/help/waits/PWAIT_DIRECTLOGCONSUMER_GETNEXT
N'PWAIT_EXTENSIBILITY_CLEANUP_TASK', -- https://www.sqlskills.com/help/waits/PWAIT_EXTENSIBILITY_CLEANUP_TASK
N'QDS_PERSIST_TASK_MAIN_LOOP_SLEEP', -- https://www.sqlskills.com/help/waits/QDS_PERSIST_TASK_MAIN_LOOP_SLEEP
N'QDS_ASYNC_QUEUE', -- https://www.sqlskills.com/help/waits/QDS_ASYNC_QUEUE
N'QDS_CLEANUP_STALE_QUERIES_TASK_MAIN_LOOP_SLEEP',
-- https://www.sqlskills.com/help/waits/QDS_CLEANUP_STALE_QUERIES_TASK_MAIN_LOOP_SLEEP
N'QDS_SHUTDOWN_QUEUE', -- https://www.sqlskills.com/help/waits/QDS_SHUTDOWN_QUEUE
N'REDO_THREAD_PENDING_WORK', -- https://www.sqlskills.com/help/waits/REDO_THREAD_PENDING_WORK
N'REQUEST_FOR_DEADLOCK_SEARCH', -- https://www.sqlskills.com/help/waits/REQUEST_FOR_DEADLOCK_SEARCH
N'RESOURCE_QUEUE', -- https://www.sqlskills.com/help/waits/RESOURCE_QUEUE
N'SERVER_IDLE_CHECK', -- https://www.sqlskills.com/help/waits/SERVER_IDLE_CHECK
N'SLEEP_BPOOL_FLUSH', -- https://www.sqlskills.com/help/waits/SLEEP_BPOOL_FLUSH
N'SLEEP_DBSTARTUP', -- https://www.sqlskills.com/help/waits/SLEEP_DBSTARTUP
N'SLEEP_DCOMSTARTUP', -- https://www.sqlskills.com/help/waits/SLEEP_DCOMSTARTUP
N'SLEEP_MASTERDBREADY', -- https://www.sqlskills.com/help/waits/SLEEP_MASTERDBREADY
N'SLEEP_MASTERMDREADY', -- https://www.sqlskills.com/help/waits/SLEEP_MASTERMDREADY
N'SLEEP_MASTERUPGRADED', -- https://www.sqlskills.com/help/waits/SLEEP_MASTERUPGRADED
N'SLEEP_MSDBSTARTUP', -- https://www.sqlskills.com/help/waits/SLEEP_MSDBSTARTUP
N'SLEEP_SYSTEMTASK', -- https://www.sqlskills.com/help/waits/SLEEP_SYSTEMTASK
N'SLEEP_TASK', -- https://www.sqlskills.com/help/waits/SLEEP_TASK
N'SLEEP_TEMPDBSTARTUP', -- https://www.sqlskills.com/help/waits/SLEEP_TEMPDBSTARTUP
N'SNI_HTTP_ACCEPT', -- https://www.sqlskills.com/help/waits/SNI_HTTP_ACCEPT
N'SOS_WORK_DISPATCHER', -- https://www.sqlskills.com/help/waits/SOS_WORK_DISPATCHER
N'SP_SERVER_DIAGNOSTICS_SLEEP', -- https://www.sqlskills.com/help/waits/SP_SERVER_DIAGNOSTICS_SLEEP
N'SQLTRACE_BUFFER_FLUSH', -- https://www.sqlskills.com/help/waits/SQLTRACE_BUFFER_FLUSH
N'SQLTRACE_INCREMENTAL_FLUSH_SLEEP', -- https://www.sqlskills.com/help/waits/SQLTRACE_INCREMENTAL_FLUSH_SLEEP
N'SQLTRACE_WAIT_ENTRIES', -- https://www.sqlskills.com/help/waits/SQLTRACE_WAIT_ENTRIES
N'VDI_CLIENT_OTHER', -- https://www.sqlskills.com/help/waits/VDI_CLIENT_OTHER
N'WAIT_FOR_RESULTS', -- https://www.sqlskills.com/help/waits/WAIT_FOR_RESULTS
N'WAITFOR', -- https://www.sqlskills.com/help/waits/WAITFOR
N'WAITFOR_TASKSHUTDOWN', -- https://www.sqlskills.com/help/waits/WAITFOR_TASKSHUTDOWN
N'WAIT_XTP_RECOVERY', -- https://www.sqlskills.com/help/waits/WAIT_XTP_RECOVERY
N'WAIT_XTP_HOST_WAIT', -- https://www.sqlskills.com/help/waits/WAIT_XTP_HOST_WAIT
N'WAIT_XTP_OFFLINE_CKPT_NEW_LOG', -- https://www.sqlskills.com/help/waits/WAIT_XTP_OFFLINE_CKPT_NEW_LOG
N'WAIT_XTP_CKPT_CLOSE', -- https://www.sqlskills.com/help/waits/WAIT_XTP_CKPT_CLOSE
N'XE_DISPATCHER_JOIN', -- https://www.sqlskills.com/help/waits/XE_DISPATCHER_JOIN
N'XE_DISPATCHER_WAIT', -- https://www.sqlskills.com/help/waits/XE_DISPATCHER_WAIT
N'XE_TIMER_EVENT' -- https://www.sqlskills.com/help/waits/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],
CAST ('https://www.sqlskills.com/help/waits/' + MAX ([W1].[wait_type]) as XML) AS [Help/Info URL]
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
Figure-2: Primary Resources Wait
Top Wait Types and Their Meaning
|
Wait Type
|
Causes
|
Fixing
|
|
CXPACKET
|
- SQL Server runs a query using multiple CPUs (parallelism) and some threads have to wait for others to finish.
|
- Tune queries, check parallelism in execution plan, check indexes.
- Check Cost Threshold/MAXDOP settings
|
|
PAGEIOLATCH_*
|
- SQL Server needs to read data from disk because the data is not already in memory.
|
- Tune queries, check indexes, check logical reads.
- Improve disk performance, add RAM
|
|
LCK_M_XX
|
- Locking so look for long-running queries.
|
- Tune queries, check indexes
|
|
SOS_SCHEDULER_YIELD
|
|
- Tune queries, add CPU resources
|
|
LATCH_*
|
|
- Tune queries, check parallelism in execution plan, check indexes.
|
Final Words
Understanding SQL Server wait types is like learning to listen to your server’s heartbeat. These waits do not just point to problems—they offer valuable insights into where your system spends its time and why. By paying attention to these quiet signals, you can proactively identify performance bottlenecks and tune your environment with precision. So the next time your SQL Server seems slow, do not just look at CPU or memory—listen to the waits. They are always whispering… are you listening?
References
Going Further
If SQL Server is your thing and you enjoy learning real-world tips, tricks, and performance hacks—you are going to love my training sessions too!
Need results fast? I am also available for 1-on-1 consultancy to help you troubleshoot and fix your database performance issues.
Let’s make your SQL Server to take your business Challenge!
For any queries, mail to mamehedi.hasan[at]gmail.com.