Configure the max worker threads Server Configuration Option
This topic describes how to configure the max worker threads server configuration option in SQL Server by using SQL Server Management Studio or Transact-SQL. The max worker threads option configures the number of worker threads that are available to SQL Server processes. SQL Server uses the native thread services of the operating systems so that one or more threads support each network that SQL Server supports simultaneously, another thread handles database checkpoints, and a pool of threads handles all users. The default value for max worker threads is 0. This enables SQL Server to automatically configure the number of worker threads at startup. The default setting is best for most systems. However, depending on your system configuration, setting max worker threads to a specific value sometimes improves performance.
In This Topic
Before you begin:
To configure the max worker threads option, using:
Before You Begin
Limitations and Restrictions
- When the actual number of query requests is less than the amount set in max worker threads, one thread handles each query request. However, if the actual number of query request exceeds the amount set in max worker threads, SQL Server pools the worker threads so that the next available worker thread can handle the request.
This option is an advanced option and should be changed only by an experienced database administrator or certified SQL Server professional. If you suspect that there is a performance problem, it is probably not the availability of worker threads. The cause is more likely something like I/O that is causing the worker threads to wait. It is best to find the root cause of a performance issue before you change the max worker threads setting.
Thread pooling helps optimize performance when large numbers of clients are connected to the server. Usually, a separate operating system thread is created for each query request. However, with hundreds of connections to the server, using one thread per query request can consume large amounts of system resources. The max worker threads option enables SQL Server to create a pool of worker threads to service a larger number of query requests, which improves performance.
The following table shows the automatically configured number of max worker threads for various combinations of CPUs and versions of SQL Server.
Number of CPUs 32-bit computer 64-bit computer <= 4 processors 256 512 8 processors 288 576 16 processors 352 704 32 processors 480 960 64 processors 736 1472 128 processors 4224 4480 256 processors 8320 8576
Using the following formula:
Number of CPUs 32-bit computer 64-bit computer <= 4 processors 256 512 > 4 processors 256 + ((logical CPU's - 4) * 8) 512 + ((logical CPU's - 4) * 16)
SQL Server can no longer be installed on a 32-bit operating system. 32-bit computer values are listed for the assistance of customers running SQL Server 2014 (12.x) and earlier. We recommend 1024 as the maximum number of worker threads for an instance of SQL Server that is running on a 32-bit computer.
For recommendations on using more than 64 CPUs, refer to Best Practices for Running SQL Server on Computers That Have More Than 64 CPUs.
When all worker threads are active with long running queries, SQL Server might appear unresponsive until a worker thread completes and becomes available. Although this is not a defect, it can sometimes be undesirable. If a process appears to be unresponsive and no new queries can be processed, then connect to SQL Server using the dedicated administrator connection (DAC), and kill the process. To prevent this, increase the number of max worker threads.
The max worker threads server configuration option does not take into account threads that are required for all the system tasks such as Availibility Groups, Service Broker, Lock Manager, and others. If the number of threads configured are being exceeded, the following query will provide information about the system tasks that have spawned the additional threads.
SELECT s.session_id, r.command, r.status, r.wait_type, r.scheduler_id, w.worker_address, w.is_preemptive, w.state, t.task_state, t.session_id, t.exec_context_id, t.request_id FROM sys.dm_exec_sessions AS s INNER JOIN sys.dm_exec_requests AS r ON s.session_id = r.session_id INNER JOIN sys.dm_os_tasks AS t ON r.task_address = t.task_address INNER JOIN sys.dm_os_workers AS w ON t.worker_address = w.worker_address WHERE s.is_user_process = 0;
Execute permissions on sp_configure with no parameters or with only the first parameter are granted to all users by default. To execute sp_configure with both parameters to change a configuration option or to run the RECONFIGURE statement, a user must be granted the ALTER SETTINGS server-level permission. The ALTER SETTINGS permission is implicitly held by the sysadmin and serveradmin fixed server roles.
Using SQL Server Management Studio
To configure the max worker threads option
In Object Explorer, right-click a server and select Properties.
Click the Processors node.
In the Max worker threads box, type or select a value from 128 through 32,767.
Use the max worker threads option to configure the number of worker threads available to SQL Server processes. The default setting for max worker threads is best for most systems. However, depending on your system configuration, setting max worker threads to a smaller value sometimes improves performance. See Recommendations in this page for more information.
To configure the max worker threads option
Connect to the Database Engine.
From the Standard bar, click New Query.
Copy and paste the following example into the query window and click Execute. This example shows how to use sp_configure to configure the
max worker threadsoption to
USE AdventureWorks2012 ; GO EXEC sp_configure 'show advanced options', 1; GO RECONFIGURE ; GO EXEC sp_configure 'max worker threads', 900 ; GO RECONFIGURE; GO
Follow Up: After you configure the max worker threads option
The change will take effect immediately after executing RECONFIGURE, without requiring the Database Engine to restart.