"But It Worked In Development!" - 3 Hard SQL Server Problems
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Warning: this is not an introductory session. These are going to be tough problems. You've been performance tuning queries and indexes for a few years, but lately, you've been running into problems you can't explain. Could it be RESOURCE_SEMAPHORE, THREADPOOL, or lock escalation? These problems only pop up under heavy load or concurrency, so they're very hard to detect in a development environment. In a very fast-paced session, I'll show these three performance problems pop up under load. I won't be able to teach you how to fix them for good - not inside the span of 75 minutes - but at least you'll be able to recognize the symptoms when they strike, and I'll show you where to go to learn more.
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"But It Worked In Development!" - 3 Hard SQL Server Problems
- 6. Agenda About my server & workload Problem #1: RESOURCE_SEMAPHORE Problem #2: THREADPOOL Problem #3: LOCK ESCALATION
- 7. SQL2016A VM with 4 cores, 8GB RAM Max server memory: 6GB RAM Stack Overflow database: 100GB dbo.Users table: <1GB
- 9. Symptoms RESOURCE_SEMAPHORE waits present Page life expectancy drops (sometimes without long-running-queries happening) SQLServer: Memory Manager – Memory Grants Pending counter goes > 0 Queries take an unpredictably long time to start
- 10. Today’s fix The query: SELECT TOP 250 * FROM dbo.Users ORDER BY Reputation DESC; The fix: • Get just the fields we need • Get less rows
- 11. Where to learn more Find queries with high memory grants: sp_BlitzCache @SortOrder = 'memory grant' Bonus points: train developers to watch this in dev, spotting high grant queries before going to production Paul White: Sorting, Row Goals, and TOP 100 http://sqlblog.com/blogs/paul_white/archive/2010/08/27/sort ing-row-goals-and-the-top-100-problem.aspx
- 13. Symptoms Apps, users can’t connect to the SQL Server Monitoring tools show gaps in history with no metrics SQL Server error log doesn’t show anything unusual
- 14. Today’s fix Create an index to let the readers go past: CREATE INDEX IX_Reputation ON dbo.Users (Reputation) INCLUDE (DisplayName, Location); Other great options: • Commit/shorten your transactions, or • Consider different isolation levels, or • Ease up on app server quantity & workloads
- 15. Where to learn more Enable & use the Dedicated Admin Connection: https://www.mssqltips.com/sqlservertip/1801/enable-sql- server-dedicated-administrator-connection/ When it hits, train someone to log in and save sp_BlitzWho or sp_WhoIsActive results to a table Use sp_BlitzIndex to find “aggressive indexes” – indexes with long lock waits, and tune those tables
- 16. Problem 3 Demo: LOCK ESCALATION(which isn’t usually typed in all capitals, but we’re on a roll here)
- 17. Symptoms Production & development have different data sizes Queries that worked fine suddenly start blocking Queries that take wildly variable workloads: • Data warehouse nightly loads • User-driven batch processing Varying execution plans for writes (narrow vs wide plans for different parameters)
- 18. Today’s fix If writers are blocking readers (or vice versa), let’s try Read Committed Snapshot Isolation (RCSI): ALTER DATABASE [StackOverflow] SET READ_COMMITTED_SNAPSHOT ON WITH NO_WAIT; Other great options: • Work in bigger batches (update everything at once), or • Purposely work in small batches, committing as you go to avoid triggering escalation, or • Consider different isolation levels
- 19. Where to learn more Michael J. Swart on batch coding: http://michaeljswart.com/2014/09/take-care-when-scripting- batches/ SQLCAT team’s Fast Ordered Deletes technique, archived by John Sansom: https://www.johnsansom.com/fast-sql-server-delete/ Support KB #323630 on lock escalation: https://support.microsoft.com/en-us/help/323630/
- 20. As data gets bigger, so do your problems.