Dynamic Memory Management

The default memory management behavior of the Microsoft SQL Server Database Engine is to acquire as much memory as it needs without creating a memory shortage on the system. The Database Engine does this by using the Memory Notification APIs in Microsoft Windows.

Virtual address space of SQL Server can be divided into two distinct regions: space occupied by the buffer pool and the rest. If AWE mechanism is enabled, the buffer pool may reside in AWE mapped memory, providing additional space for database pages.

The buffer pool serves as a primary memory allocation source of SQL Server. External components that reside inside SQL Server process, such as COM objects, and not aware of the SQL Server memory management facilities, use memory outside of the virtual address space occupied by the buffer pool.

When SQL Server starts, it computes the size of virtual address space for the buffer pool based on a number of parameters such as amount of physical memory on the system, number of server threads and various startup parameters. SQL Server reserves the computed amount of its process virtual address space for the buffer pool, but it acquires (commits) only the required amount of physical memory for the current load.

The instance then continues to acquire memory as needed to support the workload. As more users connect and run queries, SQL Server acquires the additional physical memory on demand. A SQL Server instance continues to acquire physical memory until it either reaches its max server memory allocation target or Windows indicates there is no longer an excess of free memory; it frees memory when it has more than the min server memory setting, and Windows indicates that there is a shortage of free memory.

As other applications are started on a computer running an instance of SQL Server, they consume memory and the amount of free physical memory drops below the SQL Server target. The instance of SQL Server adjusts its memory consumption. If another application is stopped and more memory becomes available, the instance of SQL Server increases the size of its memory allocation. SQL Server can free and acquire several megabytes of memory each second, allowing it to quickly adjust to memory allocation changes.