Troubleshooting Guide: SQL Server Error 522 — “The WAITFOR Thread Was Evicted”

Why This Error Happens

This error occurs when SQL Server’s scheduler forcibly evicts a worker thread that is executing a WAITFOR statement, typically because the system is under severe memory or scheduler pressure. It commonly happens when SQLOS needs to reclaim threads for higher-priority tasks, when the max worker thread pool is exhausted, or when a resource governor limit is being enforced. Corrupted or misconfigured tempdb files and insufficient memory allocation for SQL Server on Windows Server 2025 can also trigger this eviction behavior.

Step-by-Step Resolution Guide

Step 1: Check SQL Server Error Logs and Scheduler Health

• Open SQL Server Management Studio (SSMS) and run:

  EXEC sp_readerrorlog 0, 1, 'WAITFOR';

• Review C:\Program Files\Microsoft SQL Server\MSSQL17.MSSQLSERVER\MSSQL\Log\ERRORLOG for repeated eviction messages.
• Cross-reference with sys.dm_os_schedulers to identify CPU pressure:

  SELECT scheduler_id, cpu_id, status, runnable_tasks_count
  FROM sys.dm_os_schedulers
  WHERE status = 'VISIBLE ONLINE';

Step 2: Increase Worker Thread and Memory Allocation

• In SSMS, right-click the server instance → Properties → Processors tab.
• Increase Max Worker Threads from the default (0 = auto) to a higher fixed value (e.g., 2048) if thread starvation is confirmed.
• Navigate to Properties → Memory tab and verify Max Server Memory is not set too restrictively (leave at least 4–6 GB for the OS on Windows Server 2025).
• Restart the SQL Server service via services.msc or:

  Restart-Service -Name 'MSSQLSERVER' -Force

Step 3: Rebuild or Reconfigure tempdb

• Check the number and size of tempdb files:

  SELECT name, size, growth FROM tempdb.sys.database_files;

• Ensure tempdb files are located on fast storage (NVMe/SSD) and evenly sized, e.g.:

  ALTER DATABASE tempdb MODIFY FILE (NAME = tempdev, SIZE = 2048MB, FILEGROWTH = 512MB);

• If corruption is suspected, stop SQL Server, delete tempdb files manually, and let SQL Server recreate them on next startup.

Step 4: Apply Latest Cumulative Update

• Check current build version:

  SELECT @@VERSION;

• Download and install the latest SQL Server 2025 Cumulative Update (CU) from the Microsoft Update Catalog.
• Reboot the Windows Server 2025 / Windows 11 host after installation to ensure kernel-mode drivers and SQLOS scheduler patches are applied.

Alternative or Verification Steps

• After applying fixes, run a stress test using WAITFOR DELAY '00:00:10' in a loop across multiple sessions to confirm no eviction errors appear in the error log.
• If the issue persists, enable Trace Flag 8047 (NUMA-aware thread allocation) at startup and monitor sys.dm_os_wait_stats for reduced SOS_SCHEDULER_YIELD waits.