Uwe Ricken at SQL in the City 2016. Waits, as they’re known in the SQL Server world, indicate that a worker thread inside SQL Server is waiting for a resource to become available before it can proceed with executing. They’re often a major source of performance issues. In this session, we’ll walk through an optimal performance troubleshooting process for a variety of scenarios, and illustrate both the strengths and weaknesses of using a waits-only approach to troubleshooting.

1. Analyzing SQL Server wait
stats
Discover why SQL Server is
waiting

2. Get in touch
http://www.sqlmaster.de
uwe.ricken@db-berater.de
@dbberater
Uwe Ricken
#SQLintheCityUS

3. The Microsoft Certified Solutions Master (MCSM) and Microsoft Certified Master
(MCM) programs recognize the most experienced IT professionals who demonstrate
and validate their deep technical skills to successfully design and implement technical
solutions in a complex business environment.
Microsoft Certified Solutions Expert (MCSE) certifications are designed to recognize IT
professionals who can design and build solutions across multiple technologies, both on-
premises and in the cloud.
The Microsoft Most Valuable Professional (MVP) Award is our way of saying thank you
to exceptional, independent community leaders who share their passion, technical
expertise, and real-world knowledge of Microsoft products with others.
Uwe Ricken
db Berater GmbH

4. AGENDA
• what is a WAIT?
• what is a QUEUE?
• when SQL Server waits
• common Wait Stats and reasons

5. When SQL Server waits
depiction: http://rusanu.com/2013/08/01/understanding-how-sql-server-executes-a-query/

6. What is a „Wait“?
• A „wait“ occurs when a thread needs a
resource or runs out of time
– A lock on a page prevents further access
– A data page must be read from the disk
– Results from a parallel query
– …
• A waiting thread runs into a “queue”

7. What is a „Queue“?
• A „queue“ holds one or more waiting threads
• The “suspended” queue holds threads which
are waiting for resources
• The “runnable” queue holds threads which are
waiting for CPU time
• SQL Server provides DMV’s for the monitoring
of the status

8. When SQL Server waits
• sys.dm_os_wait_stats
– Information about waits which have been
executed!
• sys.dm_os_waiting_tasks
– Information about threads which are currently
waiting for resources.

9. When SQL Server waits
running
suspendedrunnable
60: RUNNING
54: WAITFOR
56: ASYNC_...
68: LCK_M_S
70: PAGELA..
55: RUNNABLE
59: RUNNABLE
72: RUNNABLE
51: RUNNABLE

10. When SQL Server waits
running
suspendedrunnable
55: RUNNING
>4ms
54: WAITFOR
56: ASYNC_...
70: PAGELA..
59: RUNNABLE
72: RUNNABLE
51: RUNNABLE
68: RUNNABLE

11. When SQL Server waits
running
suspendedrunnable
59: RUNNING
54: WAITFOR
56: ASYNC_...
70: PAGELA..
72: RUNNABLE
51: RUNNABLE
68: RUNNABLE
55: RUNNABLE
Quantum: 4ms
SOS_SCHEDULER_YIELD

12. Most common wait types
• CXPACKET
• ASYNC_NETWORK_IO
• ASYNC_IO_COMPLETION
• PAGELATCH_XX
• PAGEIOLATCH_XX
• SOS_SCHEDULER_YIELD
• WRITELOG
• THREADPOOL

13. CXPACKET
• occurs when a parallel query is executing
– Examine the long-running parallel query and
tune it if necessary.
– To ensure “efficient” parallelism, check that the
statistics on tables and indexes used by the
query are up to date.

14. CXPACKET
Transaction
1.000 records
1.000 records
1.000 records
1.000 records
CONTROLLER THREAD

15. Demo
#SQLintheCityUS

16. ASYNC_NETWORK_IO
• Your application code may not be
retrieving data correctly!
• There are very large data sets being
requested by the client!
• An application is requesting large amounts
of data!

17. Demo
#SQLintheCityUS

18. ASYNC_IO_COMPLETION
• This occurs when SQL Server is waiting for
asynchronous I/O operations to finish
• Look for additional waits in the top ranking
– IO_COMPLETION
– LOGMGR
– WRITELOG
– PAGEIOLATCH_*

19. Demo
#SQLintheCityUS

20. PAGEIOLATCH_??
• Buffer Latch for a page object on the disk
– DT (destroy)
– EX (exclusive)
– KP (keep)
– SH (shared)
– UP (update)

21. Demo
#SQLintheCityUS

22. PAGELATCH_??
• PAGELATCH_* latches are lightweight,
non-configurable internal locks used by
SQL Server’s storage engine to protect
concurrent access to pages in the
BUFFER POOL

23. Demo
#SQLintheCityUS

24. SOS_SCHEDULER_YIELD
• This common wait isn’t necessarily a
problem
• It indicates that SQL Server CPU
scheduling is working efficiently.
– RUNNING | SUSPENDED | RUNNABLE

25. Demo
#SQLintheCityUS

26. WRITELOG
• This wait occurs when data in the log cache is
being flushed to disk.
• Whenever a data page is updated, it’s written
to the buffer cache and the log cache.
• Data in the log cache is then written to the
transaction log file on the physical disk once
the transaction is complete.

27. Demo
#SQLintheCityUS

28. THREADPOOL
• SQL Server assigns a scheduler to each CPU
core to manage internal threads.
• Some of these threads are worker threads that are
assigned to query requests on a one-to-one basis.
• If every worker thread is busy, additional requests
can't be assigned straight away.
• SQL Server attempts to manage requests more
efficiently by creating a pool of worker threads, so
the next available worker thread from the pool will
pick up the task.
• If every worker thread remains busy, SQL Server
can't accept new requests and will hang.
Cores 32 bit 64 bit
<=4 256 512
5 – 8 288 576
9 – 16 352 704
32 480 960

29. Demo
#SQLintheCityUS

30. Want to try the tools you’ve just seen?
Head to the Hands-on Labs
#SQLintheCityUS