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Server configuration: cost threshold for parallelism

  • Članak

Applies to: SQL Server

This article describes how to configure the cost threshold for parallelism server configuration option in SQL Server by using SQL Server Management Studio or Transact-SQL. The cost threshold for parallelism option specifies the threshold at which SQL Server creates and runs parallel plans for queries. SQL Server creates and runs a parallel plan for a query only when the estimated cost to run a serial plan for the same query is higher than the value set in cost threshold for parallelism. The cost refers to an estimated cost required to run the serial plan on a specific hardware configuration, and isn't a unit of time. The cost threshold for parallelism option can be set to any value from 0 through 32767.

Limitations

The cost refers to an abstracted unit of cost and not a unit of estimated time. Only set cost threshold for parallelism on symmetric multiprocessors.

SQL Server ignores the cost threshold for parallelism value under the following conditions:

  • Your computer has only one logical processor.

  • Only a single logical processor is available to SQL Server because of the affinity mask configuration option.

  • The max degree of parallelism option is set to 1.

A logical processor is the basic unit of processor hardware that allows the operating system to dispatch a task or execute a thread context. Each logical processor can execute only one thread context at a time. The processor core is the circuitry that decodes and executes instructions. A processor core might contain one or more logical processors. The following Transact-SQL query can be used for obtaining CPU information for the system.

SELECT (cpu_count / hyperthread_ratio) AS PhysicalCPUs,
       cpu_count AS logicalCPUs
FROM sys.dm_os_sys_info;

Recommendations

This option is an advanced option and should be changed only by an experienced database administrator or certified SQL Server professional.

In certain cases, a parallel plan might be chosen even though the query's cost plan is less than the current cost threshold for parallelism value. This can happen because the decision to use a parallel or serial plan is based on a cost estimate provided earlier in the optimization process. For more information, see the Query Processing Architecture Guide.

While the default value of 5 is adequate for most systems, a different value might be appropriate. Perform application testing with higher and lower values if needed to optimize application performance.

Permissions

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.

Use SQL Server Management Studio

  1. In Object Explorer, right-click a server and select Properties.

  2. Select the Advanced node.

  3. Under Parallelism, change the cost threshold for parallelism option to the value you want. Type or select a value from 0 to 32767.

Use Transact-SQL

This example shows how to use sp_configure to set the value of the cost threshold for parallelism option to 10.

USE master;
GO

EXECUTE sp_configure 'show advanced options', 1;
GO

RECONFIGURE;
GO

EXECUTE sp_configure 'cost threshold for parallelism', 10;
GO

RECONFIGURE;
GO

EXECUTE sp_configure 'show advanced options', 0;
GO

RECONFIGURE;
GO

For more information, see Server configuration options.

Follow up: After you configure the cost threshold for parallelism option

The setting takes effect immediately without restarting the server.

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