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Server configuration: max degree of parallelism

  • Artikel

Applies to: SQL Server

This article describes how to configure the max degree of parallelism (MAXDOP) server configuration option in SQL Server by using SQL Server Management Studio, Azure Data Studio, or Transact-SQL. When an instance of SQL Server runs on a computer that has more than one microprocessor or CPU, the Database Engine detects whether parallelism can be used. The degree of parallelism sets the number of processors employed to run a single statement, for each parallel plan execution. You can use the max degree of parallelism option to limit the number of processors to use in parallel plan execution. For more detail on the limit set by max degree of parallelism, see the Considerations section in this page. SQL Server considers parallel execution plans for queries, index data definition language (DDL) operations, parallel inserts, online alter column, parallel stats collection, and static and keyset-driven cursor population.

SQL Server 2019 (15.x) introduced automatic recommendations for setting the max degree of parallelism server configuration option based on the number of processors available during the installation process. The setup user interface allows you to either accept the recommended settings or enter your own value. For more information, see Database Engine Configuration - MaxDOP page.

In Azure SQL Database and Azure SQL Managed Instance, the default MAXDOP setting for each new single database, elastic pool database, and managed instance is 8. In Azure SQL Database, the MAXDOP database-scoped configuration is set to 8. In Azure SQL Managed Instance, the max degree of parallelism server configuration option is set to 8.

For more on MAXDOP in Azure SQL Database, see Configure the max degree of parallelism (MAXDOP) in Azure SQL Database.

Considerations

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

If the affinity mask option isn't set to the default, it might restrict the number of processors available to SQL Server on symmetric multiprocessing (SMP) systems.

Setting max degree of parallelism to 0 allows SQL Server to use all the available processors up to 64 processors. However, this isn't the recommended value for most cases. For more information on the recommended values for max degree of parallelism, see the Recommendations section in this page.

To suppress parallel plan generation, set max degree of parallelism to 1. Set the value to a number from 1 to 32,767 to specify the maximum number of processor cores that can be used during a single query execution. If a value greater than the number of available processors is specified, the actual number of available processors is used. If the computer has only one processor, the max degree of parallelism value is ignored.

The max degree of parallelism limit is set per task. It isn't a per request or per query limit. This means that during a parallel query execution, a single request can spawn multiple tasks up to the MAXDOP limit, and each task uses one worker and one scheduler. For more information, see the Scheduling parallel tasks section in the Thread and task architecture guide.

You can override the max degree of parallelism server configuration value:

Index operations that create or rebuild an index, or that drop a clustered index, can be resource intensive. You can override the max degree of parallelism value for index operations by specifying the MAXDOP index option in the index statement. The MAXDOP value is applied to the statement at execution time and isn't stored in the index metadata. For more information, see Configure Parallel Index Operations.

In addition to queries and index operations, this option also controls the parallelism of DBCC CHECKTABLE, DBCC CHECKDB, and DBCC CHECKFILEGROUP. You can disable parallel execution plans for these statements by using Trace Flag 2528. For more information, see Trace Flag 2528.

SQL Server 2022 (16.x) introduced Degree of Parallelism (DOP) Feedback, a new feature to improve query performance by identifying parallelism inefficiencies for repeating queries, based on elapsed time and waits. DOP feedback is part of the intelligent query processing family of features, and addresses suboptimal usage of parallelism for repeating queries. For information about DOP feedback, visit Degree of parallelism (DOP) feedback.

Recommendations

In SQL Server 2016 (13.x) and later versions, during service startup if the Database Engine detects more than eight physical cores per NUMA node or socket at startup, soft-NUMA nodes are created automatically by default. The Database Engine places logical processors from the same physical core into different soft-NUMA nodes. The recommendations in the following table are aimed at keeping all the worker threads of a parallel query within the same soft-NUMA node. This improves the performance of the queries and distribution of worker threads across the NUMA nodes for the workload. For more information, see Soft-NUMA (SQL Server).

In SQL Server 2016 (13.x) and later versions, use the following guidelines when you configure the max degree of parallelism server configuration value:

Server configuration Number of processors Guidance
Server with single NUMA node Less than or equal to eight logical processors Keep MAXDOP at or under the # of logical processors
Server with single NUMA node Greater than eight logical processors Keep MAXDOP at 8
Server with multiple NUMA nodes Less than or equal to 16 logical processors per NUMA node Keep MAXDOP at or under the # of logical processors per NUMA node
Server with multiple NUMA nodes Greater than 16 logical processors per NUMA node Keep MAXDOP at half the number of logical processors per NUMA node with a MAX value of 16

NUMA node in the previous table refers to soft-NUMA nodes automatically created by SQL Server 2016 (13.x) and higher versions, or hardware-based NUMA nodes if soft-NUMA is disabled.

Use these same guidelines when you set the max degree of parallelism option for Resource Governor workload groups. For more information, see CREATE WORKLOAD GROUP.

SQL Server 2014 and earlier versions

From SQL Server 2008 (10.0.x) through SQL Server 2014 (12.x), use the following guidelines when you configure the max degree of parallelism server configuration value:

Server configuration Number of processors Guidance
Server with single NUMA node Less than or equal to eight logical processors Keep MAXDOP at or under the # of logical processors
Server with single NUMA node Greater than eight logical processors Keep MAXDOP at 8
Server with multiple NUMA nodes Less than or equal to eight logical processors per NUMA node Keep MAXDOP at or under the # of logical processors per NUMA node
Server with multiple NUMA nodes Greater than eight logical processors per NUMA node Keep MAXDOP at 8

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 or Azure Data Studio

In Azure Data Studio, install the Database Admin Tool Extensions for Windows extension, or use the following T-SQL method.

These options change the MAXDOP for the instance.

  1. In Object Explorer, right-click the desired instance and select Properties.

  2. Select the Advanced node.

  3. In the Max Degree of Parallelism box, select the maximum number of processors to use in parallel plan execution.

Use Transact-SQL

  1. Connect to the Database Engine with SQL Server Management Studio or Azure Data Studio.

  2. From the Standard bar, select New Query.

  3. Copy and paste the following example into the query window and select Execute. This example shows how to use sp_configure to configure the max degree of parallelism option to 16.

    USE master;
GO

EXECUTE sp_configure 'show advanced options', 1;
GO

RECONFIGURE WITH OVERRIDE;
GO

EXECUTE sp_configure 'max degree of parallelism', 16;
GO

RECONFIGURE WITH OVERRIDE;
GO

EXECUTE sp_configure 'show advanced options', 0;
GO

RECONFIGURE;
GO

For more information, see Server configuration options.

Follow up: After you configure the max degree of parallelism option

The setting takes effect immediately without restarting the server.

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