This document discusses wait statistics and wait analysis in SQL Server. It defines waits as occurring when a process requests a resource that is unavailable. It provides an overview of wait types categorized as background, external, or queue waits. The document demonstrates how to view wait statistics using DMVs and third party tools. It provides translations and guidelines for some common wait types like CXPACKET, PAGEIOLATCH, and LATCH. It concludes with additional resources for learning about waits.

1. What Are You Waiting For?
Performing Wait Stat Analysis

2. Introduction
Jason e: jstrate@pragmaticworks.com
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Resources jasonstrate.com/go/Waits
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3. What Are You Waiting For?
You…
• SQL Server
• Developers
• DBAs

4. Agenda
Defining Collecting
Viewing Reviewing

5. Agenda
Defining Collecting
Viewing Reviewing

6. Wait, wait… I’ll Tell You
Waits: Resource waits occur when a worker
requests access to a resource that is not
available because the resource is being used by
some other worker or is not yet available.

7. So What?!

8. Wait Statistics
Identity
Prioritize Measure

9. Execution Model (Simplified)
Grocery Store
• Cashier
– Scheduler
• Customer
– Query
• Checkout Line
– Queue
• Running
• Runnable
• Suspended

10. Execution Model (Simplified)
RUNNING

11. Execution Model (Simplified)
RUNNABLE RUNNING

12. Execution Model (Simplified)
RUNNABLE RUNNING
WAIT
SUSPENDED

13. Execution Model (Simplified)
RUNNABLE RUNNING
SUSPENDED

14. Execution Model (Simplified)
RUNNABLE RUNNING
WAIT TIME
SUSPENDED

15. Execution Model (Simplified)
RUNNABLE RUNNING
SUSPENDED

16. Execution Model (Simplified)
RUNNABLE RUNNING
SUSPENDED

17. Execution Model (Simplified)
RUNNABLE RUNNING
SIGNAL WAIT SUSPENDED
TIME

18. Agenda-ing
Defining Collecting
Viewing Reviewing

19. Wait DMVs
• sys.dm_os_wait_stats
– Accumulated statistics on tasks that have waited
for resources
• sys.dm_os_waiting_tasks
– Information on queued tasks waiting for
resources

20. Waits Stats

21. Waits Stats
Name of the wait type.

22. Waits Stats
Number of waits on this wait type. This counter is
incremented at the start of each wait.

23. Waits Stats
Total wait time for this wait type in milliseconds. This
time is inclusive of signal_wait_time_ms.

24. Waits Stats
Maximum wait time on this wait type.

25. Waits Stats
Difference between the time that the waiting thread
was signaled and when it started running.

26. Waiting Tasks

27. Waiting Tasks

28. Waiting Tasks
ID of the session associated with the task.

29. Waiting Tasks
Total wait time for this wait type, in milliseconds. This
time is inclusive of signal_wait_time.

30. Waiting Tasks
Name of the wait type.

31. Waiting Tasks
ID of the session that is blocking the request. If this
column is NULL, the request is not blocked, or the
session information of the blocking session is not
available

32. Waiting Tasks
Difference between the time that the waiting thread
was signaled and when it started running.

33. Demo
VIEWING WAIT STATS

34. Agenda
Defining Collecting
Viewing Reviewing

35. DMV Stats

36. Management Data Warehouse

37. Do-It-Yourself
Collect Summarize Analyze

38. Demo
DO-IT-YOURSELF

39. Agenda
Defining Collecting
Viewing Reviewing

40. Wait Types
• Tied to process
– Audit
– Buffer
– I/O
– Indexing
– Lock
– SQLOS
• Categories
– Background
– External
– Queue

41. Background Waits
CLR_SEMAPHORE CHECKPOINT_QUEUE
LAZYWRITER_SLEEP XE_TIMER_EVENT
RESOURCE_QUEUE BROKER_TO_FLUSH
SLEEP_TASKNot all inclusive
BROKER_TASK_STOP
SLEEP_SYSTEMTASK CLR_MANUAL_EVENT
WAITFOR CLR_AUTO_EVENT
LOGMGR_QUEUE XE_DISPATCHER_WAIT

42. CXPACKET
Translation
• Contention with queries the at are executing in parallel; across multiple CPUs.
Generally due to unbalanced execution across CPUs or tasks ahead of query
executing longer than expected.
Guideline
• >5% for OLTP
• >10% for data warehouses
Resolution
• Review indexing
• Tune queries
• Adjust Max Degree of parallelism
• Research cost threshold for parallelism

43. OLEDB
Translation
• Occurs when SQL Server calls the Microsoft SQL Native Client OLE DB
Provider. It indicates the duration of calls to the OLE DB provider.
Guideline
• Examine when it is a top wait and exceeds background waits. (TOP 5)
Resolution
• Identify queries with OLE DB waits with extended events or
sys.dm_os_waiting_tasks
• Check application placement and Disk secs/Read and Disk secs/Write
• Tune transactions using RPC, Distributed (Linked Server), and Full Text
Search

44. PAGEIOLATCH_x
Translation
• Contention caused by disk to memory transfer. Typically suggests disk IO
subsystem issues.
Guideline
• Examine when it is a top wait and exceeds background waits. (TOP 5)
Resolution
• Review Physical disk: disk seconds/read and Physical disk: disk seconds/write and
SQL Server Buffer Manager: Page Life Expectancy
• Review information for virtual file stats
• Reconfigure disk to improve IO throughput
• Analyze indexes to reduce need to move data to memory
• Increase memory to increase data available to SQL Server

45. PAGELATCH_x
Translation
• Contention between short term light weight synchronization objects. Latches
are not held for the duration of a transaction. Occur in the buffer pool but
unrelated to IO requests.
Guideline
• Examine when it is a top wait and exceeds background waits. (TOP 5)
Resolution
• Check tempdb configuration
• Additional data files
• Trace Flag 1118
• Troubleshoot memory pressure

46. LATCH_x
Translation
• Contention between short term light weight synchronization objects.
Latches are not held for the duration of a transaction. Non buffer pool
related.
Guideline
• Examine when it is a top wait and exceeds background waits. (TOP 5)
Resolution
• Review T-SQL generating LATCH waits using extended events.
• Examine memory items in sys.dm_os_latch_stats
• Partition table across multiple files

47. Additional Resources
Waits and Queues • http://is.gd/qWGGnd
SQLServerPedia • http://is.gd/gvD80N
SQL Server Wait Type • http://is.gd/r59976
Repository
Wait Stats Introductory • http://is.gd/khsduZ
References
Joe Sack • http://is.gd/dQWFwC
• http://is.gd/VlH71l
Management Data Warehouse • http://is.gd/xggZSA
DMVstats • http://sqldmvstats.codeplex.com

48. For more information…
Name: Jason Strate
Email: jstrate@pragmaticworks.com
Blog: www.jasonstrate.com
Resource: jasonstrate.com/go/waits
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