The document discusses SQL Server wait statistics, detailing their importance in diagnosing performance issues across various SQL Server versions. It emphasizes the evolution of SQL Server's ability to analyze waits, the integration of dynamic management objects, and provides insights into common wait types. Additionally, it highlights the necessity of correlating wait statistics with performance counters to improve database performance.

1. WAIT WATCHERS;
GAIN PERFORMANCE
INCREASES FAST!
Richard Douglas
• Sales Engineer, SQL Sentry
• Social Media: @SQLRich, LinkedIn,
• Blog Info: http://blogs.sqlsentry.com/author/RichardDouglas/;
http://SQL.RichardDouglas.co.uk

2. Tuning blog: http://www.sqlperformance.com/
E-mail ebooks@sqlsentry.com for free copies of our $10 e-books:

3. AGENDA
• Introduction
• How and why waits occur
• Looking at and deciphering waits
• Demo
• Q&A

4. YOUR HOST
• Richard Douglas
• Sales Engineer at SQL Sentry
• Blog:
http://SQL.RichardDouglas.co.uk
• Twitter: @SQLRich
• Email: RDouglas@SQLSentry.com
• Slides:
http://www.Slideshare.net/SQLRich

5. INTRODUCTION
Onceuponatime we usedto checkserver performancelike this:

6. INTRODUCTION
Onceuponatime we usedto checkserver performancelike this:

7. MOVING ON – PERFORMANCE COUNTERS
Thenwe abletolookatresources throughperformancecounters…

8. INTRODUCING WAIT STATISTICS
• In SQLServer2000Microsoftintroducedawaytolookatwhatit
waswaitingforwhen askingforresources.
• This wasimprovedsignificantlyin 2005withDynamic
ManagementObjects.
• In 2008wecoulddo even morewithExtendedEvents.
• Versions 2012& 2014haveaddedmorevalue with morewaits
tofurtherdiagnoseissues.
SQL 2000: 78 wait types
SQL 2005: 202 wait types
SQL 2008: 484 wait types
SQL 2008 R2: 490 wait types
SQL 2012: 650 wait types
SQL 2014: 772 wait types

9. THE WHOLE TRUTH AND NOTHING BUT THE TRUTH?
A BIG problem isthatbothcountersandwaitstatisticsonlytell partofa story,unless used together.

10. My Holiday
OBLIGATORY ANALOGY
• Going throughSecurity:
Queue
Bag
Scan
Body
Scan

11. My Holiday
OBLIGATORY ANALOGY
Queue
Bag
Scan
Ready
Body
Scan
• Going throughSecurity:

12. My Holiday
OBLIGATORY ANALOGY
Queue
Bag
Scan
Processed
Body
Scan
• Going throughSecurity:

13. My Holiday
OBLIGATORY ANALOGY
• Going throughSecurity:
Queue
Bag
Scan
Body
Scan
Processed

14. My Holiday
OBLIGATORY ANALOGY
• Going throughSecurity:
Queue
Bag
Scan
Body
Scan
Suspended / Ready

15. My Holiday
OBLIGATORY ANALOGY
• Going throughSecurity:
Queue
Bag
Scan
Body Scan
Processed

16. My Holiday
OBLIGATORY ANALOGY
• Going throughSecurity:
Queue
Bag
Scan
Body Scan
Shopping!

17. WHY THE SQL OS?
• The SQL OS?
SQL runs on Windows that’s an OS what is this guy talking about?
• Windows Server isn’t a highly optimized Operating System by default. It’s designed to allow
development teams to write applications that will run and allow them to compete in their field
of business.
• In other words Microsoft built Windows Server to be stable and available, not to be optimal.

18. WHY THE SQL OS? (2)
• The SQL Server team realized the limitations with a number
of the scheduling routines and decided to write their own
scheduling routines which would enhance the performance
of SQL Server.
• In 2005 the SQL OS was born.
• Any Dynamic Management Objects that start sys.dm_os_
provide an insight into the working of the SQLOS.
• Let’s see how the SQL OS controls requests for CPU time
using a simplified example on just one logical processor…

19. Single Scheduler
THREAD EXECUTION
Running Scheduler 1
55 - Running
Suspended Scheduler 1
Runnable Scheduler 1
60 - Runnable
65 - Runnable
SPID 55

20. Single Scheduler
THREAD EXECUTION
Running Scheduler 1 Suspended Scheduler 1
55 - PAGEIOLATCH_SH
Runnable Scheduler 1
60 - Runnable
65 - Runnable
SPID60

21. Single Scheduler
THREAD EXECUTION
Running Scheduler 1
60 - Running
Suspended Scheduler 1
55 - PAGEIOLATCH_SH
Runnable Scheduler 1
65 - Runnable
SPID65
SPID60

22. Single Scheduler
THREAD EXECUTION
Running Scheduler 1
65 - Running
Suspended Scheduler 1
Runnable Scheduler 1
55 - Runnable
SPID55
SPID65

23. VIEWING
WAIT STATISTICS
INFORMATION

24. VIEWING WAITS
• DynamicManagement Views
Wewill cover anumberofthesetoday.
• ExtendedEvents
Notenough timetocoverthis in anydetail today.Contactme atrdouglas@SQLSentry.comif youwouldlike tosee a sessionon thisor
anyothersubject.
• Thirdpartytools
Wehavesomegreattools,get in touchif youwanttoknowmore
athttp://www.SQLSentry.comandwatchsomefreetrainingathttp://www.SQLSentry.tv

25. DBCC SQLPERF
“GOIN’ OLD SKOOL” WITH DBCC
• DBCCSQLPERF(waitstats)
• It will showyouall possiblewaitsforyoureditioneven if theyhavenotoccurred.
• Youcanalso cleardownthe waitstatsinformationusing:
DBCCSQLPERF("sys.dm_os_wait_stats",clear)

26. Sys.dm_exec_requests
DYNAMIC MANAGEMENT VIEWS
• Waitinformation
• Tasklevel
• Veryaccurate
• Transientdata

27. sys.dm_os_wait_stats
DYNAMIC MANAGEMENT VIEWS
• wait_type – The name of the wait type.
• waiting_tasks_count – The number of waits on this wait type.
• wait_time_ms – The total wait time for this wait type in milliseconds, this includes
signal_wait_time.
• max_wait_time_ms – The maximum wait time on this wait type.
• signal_wait_time – The difference between the time the waiting thread was signalled and
when it started running (runnable queue)
• Books Online has a list of descriptions for some of the wait types, worth a look.
Big Warning from Books Online
In SQL Server the wait-time counters are bigint values and therefore are not as prone to counter
rollover as the equivalent counters in earlier versions of SQL Server.

28. Sys.dm_os_waiting_tasks
DYNAMIC MANAGEMENT VIEWS
• Similartosys.dm_exec_requestsin thatitreturnscurrentinformationratherthanaggregated.
• Will notreturninfoonsessions thataresleeping –theyarenotwaiting!
• Will returnrecordsthathaveno SessionIDandarenotinsys.dm_exec_requests.
• Youwill notseeSOS_SCHEDULER_YIELDhere, it’s notwaiting.
Moreon thislater…

29. DECIPHERING
WAIT
INFORMATION

30. COMMON WAIT TYPES
PAGEIOLATCH_*
• Latch on a memory address while data is pulled
from disk
PAGELATCH_*
• Latch on a page in memory
CXPACKET
• Query parallelism
Remember you need to correlate wait stats with
perfmon counters.
There’s a breakdown of counters and suggested
thresholds available in my presentation “The
Day After Tomorrow; Why You Need to
Baseline”
http://www.SlideShare.net/SQLRich

31. Not all waits are bad ANALOGY 2
Waiting
for elevator Ready Suspended

32. Not all waits are bad ANALOGY 2
Waiting
for elevator
Processing

33. Not all waits are bad ANALOGY 2
Waiting
for elevator
Stops at
other floors Suspended

34. Not all waits are bad ANALOGY 2
Waiting
for elevator
Stops at
other floors
Stops at
my floor
Processing
• The waits weren’t bad per se.
• There’s only a problem if the
combined ready (runnable) and
suspended states were greater then
the time it would take to walk the
stairs.
• If we could remove or reduce the
“suspended” waiting time we would
experience better performance.

35. COMMON WAIT TYPES
WRITELOG
• Writing transaction log to disk
LCK_M_*
• Waiting for a lock
SOS_SCHEDULER_YIELD
• Yielding processor time
• See signal wait

36. (Of Solace) THE QUANTUM
• Runningtaskswill be kicked offthe scheduler soothersessions in theRunnabledon’tstarve

37. Single Scheduler
THREAD EXECUTION
Running Scheduler 1
55 - Running
Suspended Scheduler 1
Runnable Scheduler 1
60 - Runnable
65 - Runnable
SPID 55

38. Single Scheduler
THREAD EXECUTION
Running Scheduler 1 Suspended Scheduler 1
55 – PAGEIOLATCH_SH
Runnable Scheduler 1
60 – Runnable
65 - Runnable
SPID60

39. Single Scheduler
THREAD EXECUTION
Running Scheduler 1
60 – Running
Suspended Scheduler 1
55 – PAGEIOLATCH_SH
Runnable Scheduler 1
65 - Runnable
SPID65
SPID60

40. Single Scheduler
THREAD EXECUTION
Running Scheduler 1
65 – Running
Suspended Scheduler 1
Runnable Scheduler 1
55 – Runnable
60 – Runnable
(SOS_Scheduler_Yield)
SPID55
SPID65

41. This happened earlier

42. THANK YOU!
• Slides will be available at http://Slideshare.net/SQLRich
• More information at:
o SQLSkills, et al
• E-mail ebooks@sqlsentry.com for free copies of our e-books:
o Just tell them where you met me
• My contact info for other questions:
o RDouglas@SQLSentry.com
o Twitter: @SQLRich
o Blogs: http://SQL.RichardDouglas.co.uk /
http://http://blogs.sqlsentry.com/author/RichardDouglas

43. SOME GREAT RESOURCES
• SQLServer2005Waitsand Queues;SQLServerBest PracticesArticle
• Recommendationsandguidelines forthe "maxdegree of parallelism"configurationoptionin SQL
Server- http://support.microsoft.com/kb/2806535
• Books:ProfessionalSQLServer2008/ 2012 InternalsandTroubleshooting
• Books:MicrosoftSQLServer2008/ 2012 Internals