This document discusses various resource bottlenecks that can occur in SQL Server including CPU, memory, I/O, network, and tempdb bottlenecks. It provides system views and metrics to identify the cause of bottlenecks and monitor resources. Specific issues covered include excessive compilation, hash/sort operations, large scans/sorts, memory leaks, storage performance, parallelism levels, and missing indexes that can lead to slow queries. The document demonstrates using SQL Profiler to troubleshoot performance issues.

1. SQL Server
Performance Troubleshooting
Dan Andrei STEFAN
Database Administrator
Synygy Europa
09.05.2013

2. Resource Bottlenecks
vs.

3. CPU
What can lead to high CPU usage?
• excessive compilation / recompilation
sys.dm_exec_query_optimizer_info
– statistics changes
– unparametrized queries
alter database {} set parameterization forced
Oracle equivalent: cursor_sharing=force
• hash / sort operators
sys.dm_db_task_space_usage
Oracle equivalent: v$sort_usage
• queries that consume cpu time
sys.dm_exec_query_stats
Oracle equivalent: v$sqlstats
• 3rd party applications / services on db server

4. CPU Tip
• How to compute the CPU % per request?
sys.dm_os_threads

5. Memory
What can lead to high memory usage or memory pressure?
• Large table / index scans
• Large sorts
• Cached plans for unparametrized queries
• High value for OS available memory
• Memory leaks
• 3rd party applications / services on db server

6. Memory Pressure

7. Memory Tip

8. I/O
What can lead to I/O bottlenecks?
• Missing indexes
• Full Table Scan / Clustered Index Scan
• Memory pressure
• Large sort operations placed on tempdb
• Poor storage infrastructure / IO subsystem
• high values for read/write latency
• Partitions not being aligned
• 3rd party applications / services on db server

9. I/O Tip
• How to find the HoBT on which sessions are waiting?
sys.dm_os_waiting_tasks
• Identify on what is page on what the transaction is currently waiting
• get page details
• query for object name

10. Network
What can lead to network bottlenecks?
• Mirroring (High Safety Mode)
• Network card / driver
– Check ProcessorInterruptsSec counter
• Network infrastructure
– switches, routers, firewalls
– Bandwidth
• DoS

11. tempdb
What can lead to tempdb bottlenecks?
sys.dm_db_file_space_usage
• Memory pressure
• temporary user tables
• Large sorts runs / work files for hash join
• Version store size
• Improper number of files
– not 1 file / cpu core, sometime it is too much (16 cores -> 16 files?!)
– SGAM, GAM, PFS contention
– Round robin allocation mechanism

12. Parallelism
Up to what level parallelism is good?
• DOP gives the number of threads per parallel operator
– 0 (default) means CPU count
– I’ve seen queries running with 81, 73, 58 threads on an 8 CPUs machine
• Too many threads
– Low CPU usage
– High values of CXPACKET
– High values of Context Switches
– Spinlocks

13. Slow running queries
Knowing about resources, what make a query slow?
• Improper statistics, leading to improper execution plan
Nested loops vs. hash join
• Improper join/filter conditions, leading to huge /very low cardinality
estimations
Histograms in SQL Server 2000/2005/2008 are only built for a single column—the first column in the set of key columns of
the statistics object.
• Missing indexes
sys.dm_db_missing_index_groups mig / sys.dm_db_missing_index_group_stats / sys.dm_db_missing_index_details
• Too many threads open
• Waits (I/O, memory, CPU, locks)
sys.dm_os_waiting_tasks
• and many, many others

14. SQL Profiler - demo
• Hopefully, it will work 
Remember Windows 95 presentation?

15. Thank you
• Slide 6 again?