max worker threads Option
Use the max worker threads option to configure the number of worker threads available to Microsoft SQL Server processes. SQL Server uses the native thread services of the Microsoft Windows 2000 and Windows Server 2003 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.
Purpose of max worker threads Option
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 request, which improves performance.
Calculating max worker threads
The default value for max worker threads, 0, allows SQL Server to automatically configure the number of worker threads at startup. This setting is best for most systems; however, depending on your system configuration, setting max worker threads to a specific value sometimes 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 |
Note
We recommend 1024 as the maximum for 32-bit SQL Server and 2048 for 64-bit SQL Server.
When the actual number of query request 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.
Upgrading from SQL Server 2000
The default setting of max worker threads in SQL Server 2000 was 255. Upgrading an instance of the SQL Server 2000 Database Engine to a newer version retains the configuration value for max worker threads. When upgrading, we recommend changing the new instance max worker threads value to 0, to allow the Database Engine to calculate the optimal number of threads.
Setting max worker threads
The max worker threads option is an advanced option. If you are using the sp_configure system stored procedure to change the setting, you can change max worker threads only when show advanced options is set to 1. The system must be restarted in order for the new setting to take effect.
Note
When all worker threads are active with long running queries, SQL Server may appear unresponsive until a worker thread completes and becomes available. Though not a defect, this 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.