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locks Option

Use the locks option to set the maximum number of available locks, thereby limiting the amount of memory the Database Engine uses for them. The default setting is 0, which allows the Database Engine to allocate and deallocate lock structures dynamically, based on changing system requirements.

When the server is started with locks set to 0, the lock manager acquires sufficient memory from the Database Engine for an initial pool of 2,500 lock structures. As the lock pool is exhausted, additional memory is acquired for the pool.

Generally, if more memory is required for the lock pool than is available in the Database Engine memory pool, and more computer memory is available (the max server memory threshold has not been reached), the Database Engine allocates memory dynamically to satisfy the request for locks. However, if allocating that memory would cause paging at the operating system level (for example, if another application is running on the same computer as an instance of SQL Server and using that memory), more lock space is not allocated. The dynamic lock pool does not acquire more than 60 percent of the memory allocated to the Database Engine. After the lock pool has reached 60 percent of the memory acquired by an instance of the Database Engine, or no more memory is available on the computer, further requests for locks generate an error.

Allowing SQL Server to use locks dynamically is the recommended configuration. However, you can set locks and override the ability of SQL Server to allocate lock resources dynamically. When locks is set to a value other than 0, the Database Engine cannot allocate more locks than the value specified in locks. Increase this value if SQL Server displays a message that you have exceeded the number of available locks. Because each lock consumes memory (96 bytes per lock), increasing this value can require increasing the amount of memory dedicated to the server.

The locks option also affects when lock escalation occurs. When locks is set to 0, lock escalation occurs when the memory used by the current lock structures reaches 40 percent of the Database Engine memory pool. When locks is not set to 0, lock escalation occurs when the number of locks reaches 40 percent of the value specified for locks.

The locks option is an advanced option. If you are using the sp_configure system stored procedure to change the setting, you can change locks only when show advanced options is set to 1. The setting takes effect after the server is restarted.

On computers that are using nonuniform memory access (NUMA), the locks setting is per NUMA node; therefore, a setting of 10,000 on a four-node computer allows 40,000 locks. The number of NUMA nodes can be determined by using the following query:

SELECT COUNT(DISTINCT parent_node_id)
FROM sys.dm_os_schedulers 
WHERE parent_node_id <> 32
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