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Server configuration: affinity I/O mask

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Applies to: SQL Server

To carry out multitasking, Windows sometimes move process threads among different processors. Although efficient from an operating system point of view, this activity can reduce SQL Server performance under heavy system loads, as each processor cache is repeatedly reloaded with data. Assigning processors to specific threads can improve performance under these conditions by eliminating processor reloads; such an association between a thread and a processor is called processor affinity.

SQL Server supports processor affinity with two affinity mask options: affinity mask (also known as CPU affinity mask) and affinity I/O mask. For more information on the affinity mask option, see Configure the affinity mask server configuration option. CPU and I/O affinity support for servers with 33 to 64 processors requires that you also use the affinity64 mask and affinity64 I/O mask server configuration options respectively.

Note

Affinity support for servers with 33 to 64 processors is only available on 64-bit operating systems.

The affinity I/O mask option binds SQL Server disk I/O to a specified subset of CPUs. In high-end SQL Server online transactional processing (OLTP) environments, this extension can enhance the performance of SQL Server threads issuing I/Os. This enhancement doesn't support hardware affinity for individual disks or disk controllers.

The value for affinity I/O mask specifies which CPUs in a multiprocessor computer are eligible to process SQL Server disk I/O operations. The mask is a bitmap in which the rightmost bit specifies the lowest-order CPU(0), the bit to its immediate left specifies the next-lowest-order CPU(1), and so on. To configure more than 32 processors, set both the affinity I/O mask and the affinity64 I/O mask.

The values for affinity I/O mask are as follows:

Bytes in mask Number of CPUs
1-byte Up to 8 CPUs
2-byte Up to 16 CPUs
3-byte Up to 24 CPUs
4-byte Up to 32 CPUs

To cover more than 32 CPUs, configure a 4-byte affinity I/O mask for the first 32 CPUs and up to a 4-byte affinity64 I/O mask for the remaining CPUs.

A 1 bit in the affinity I/O pattern specifies that the corresponding CPU is eligible to perform SQL Server disk I/O operations. A 0 bit specifies that no SQL Server disk I/O operations should be scheduled for the corresponding CPU. When all bits are set to 0, or affinity I/O mask isn't specified, SQL Server disk I/O is scheduled to any of the CPUs eligible to process SQL Server threads.

Because setting the SQL Server affinity I/O mask option is a specialized operation, use it only when necessary. In most cases, the default Windows affinity provides the best performance.

When specifying the affinity I/O mask option, you must use it with the affinity mask configuration option. Don't enable the same CPU in both the affinity I/O mask switch and the affinity mask option. The bits corresponding to each CPU should be in one of the following three states:

  • 0 in both the affinity I/O mask option and the affinity mask option.
  • 1 in the affinity I/O mask option and 0 in the affinity mask option.
  • 0 in the affinity I/O mask option and 1 in the affinity mask option.

The affinity I/O mask option is an advanced option. If you're using the sp_configure system stored procedure to change the setting, you can change affinity I/O mask only when show advanced options is set to 1. In SQL Server, reconfiguring the affinity I/O mask option requires a restart of the SQL Server instance.

Caution

Don't configure CPU affinity in the Windows operating system and also configure the affinity mask in SQL Server. These settings are attempting to achieve the same result, and if the configurations are inconsistent, you can have unpredictable results. SQL Server CPU affinity is best configured using the sp_configure system stored procedure in SQL Server.

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