Introduction SQL Server Extended Events (XEvents) are a lightweight performance monitoring system. However, misconfigured XEvent sessions—especially those capturing high-frequency events, using complex actions, or writing to slow targets—can cause significant CPU overhead and performance degradation on production servers.

Symptoms - CPU usage spikes after enabling an Extended Events session - `sqllang.dll` or `xe.dll` showing high CPU in process monitor - Query performance degrades when XEvent session is active - System Health session causing measurable overhead on busy servers - Ring buffer target filling up and causing memory pressure

Common Causes - Capturing high-frequency events like `sql_statement_completed` on a busy server - Using expensive actions like `sql_text` or `query_plan` on every event - Writing to file target on a slow disk with synchronous writes - No event filtering (predicate) capturing all events instead of specific conditions - Multiple XEvent sessions running simultaneously

Step-by-Step Fix 1. **List active Extended Events sessions": ```sql SELECT s.name AS session_name, s.create_time, s.max_memory, s.total_regular_buffer_size, s.total_buffer_size, s.dropped_event_count, s.create_time FROM sys.dm_xe_sessions s;

-- Check events and actions for each session SELECT s.name AS session_name, e.name AS event_name, a.name AS action_name FROM sys.dm_xe_sessions s JOIN sys.dm_xe_session_events se ON s.address = se.event_session_address JOIN sys.dm_xe_objects e ON se.event_name = e.name LEFT JOIN sys.dm_xe_session_event_actions sea ON se.event_session_address = sea.event_session_address AND se.event_name = sea.event_name LEFT JOIN sys.dm_xe_objects a ON sea.action_name = a.name; ```

  1. 1.**Stop the problematic session":
  2. 2.```sql
  3. 3.ALTER EVENT SESSION [HighCPU_Trace] ON SERVER STATE = STOP;

-- If you need to keep it but reduce impact, modify it ALTER EVENT SESSION [HighCPU_Trace] ON SERVER DROP EVENT sql_statement_completed;

-- Add a more specific event with predicates ALTER EVENT SESSION [HighCPU_Trace] ON SERVER ADD EVENT sqlserver.sql_statement_completed( ACTION(sqlserver.database_name, sqlserver.sql_text) WHERE sqlserver.database_name = 'mydb' AND duration > 1000000 -- Only capture queries over 1 second ); ```

  1. 1.**Configure the session for lower overhead":
  2. 2.```sql
  3. 3.CREATE EVENT SESSION [Optimized_Trace] ON SERVER
  4. 4.ADD EVENT sqlserver.sql_statement_completed(
  5. 5.SET collect_statement_text = 0 -- Don't collect text by default
  6. 6.ACTION(sqlserver.database_id, sqlserver.session_id)
  7. 7.WHERE sqlserver.database_id > 4 -- Exclude system databases
  8. 8.AND duration > 500000 -- Microseconds: only slow queries
  9. 9.)
  10. 10.ADD TARGET package0.event_file(
  11. 11.SET filename = N'C:\XEvents\Optimized_Trace.xel',
  12. 12.max_file_size = 100, -- MB
  13. 13.max_rollover_files = 4
  14. 14.)
  15. 15.WITH (
  16. 16.MAX_MEMORY = 4096 KB,
  17. 17.EVENT_RETENTION_MODE = ALLOW_SINGLE_EVENT_LOSS,
  18. 18.MAX_DISPATCH_LATENCY = 5 SECONDS,
  19. 19.TRACK_CAUSALITY = OFF
  20. 20.);
  21. 21.`
  22. 22.**Clean up completed session files":
  23. 23.```sql
  24. 24.-- Query the event file
  25. 25.SELECT
  26. 26.object_name AS event_name,
  27. 27.CAST(event_data AS XML) AS event_xml,
  28. 28.file_name
  29. 29.FROM sys.fn_xe_file_target_read_file(
  30. 30.'C:\XEvents\Optimized_Trace*.xel',
  31. 31.NULL, NULL, NULL
  32. 32.);
  33. 33.`

Prevention - Always use predicates to filter events to specific conditions - Avoid `sql_text` and `query_plan` actions in high-frequency event sessions - Set `MAX_DISPATCH_LATENCY` to 5-30 seconds to batch writes - Use `ALLOW_SINGLE_EVENT_LOSS` for production monitoring - Test XEvent sessions in staging with production-level load - Monitor CPU impact before and after enabling XEvent sessions - Disable sessions when not actively troubleshooting; do not leave them running permanently