Analyzing SQL Server wait stats, hands-on!
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The document discusses SQL Server wait statistics, explaining waits and queues, and emphasizes monitoring methods using dynamic management views (DMVs). It highlights common wait types, such as cxpacket, async_network_io, and writelog, detailing their causes and how to address them. Additionally, the document addresses the importance of efficient scheduling and thread management within SQL Server.
Analyzing SQL Server wait stats, hands-on!
- 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
- 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!
- 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_*
- 20. PAGEIOLATCH_?? • Buffer Latch for a page object on the disk – DT (destroy) – EX (exclusive) – KP (keep) – SH (shared) – UP (update)
- 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
- 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
- 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.
- 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
- 30. Want to try the tools you’ve just seen? Head to the Hands-on Labs #SQLintheCityUS