This gauge shows the total memory that SQL Server is currently using scaled against the maximum amount of memory it can use.
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By default, SQL Server dynamically manages its memory. It will request extra memory from the operating system as required to satisfy the queries being run. SQL Server also monitors the stress on the Windows memory manager and when it finds that the memory manager is having trouble satisfying the memory requests of other windows processes, SQL Server will voluntarily release some of its memory back to windows.

This dynamic memory management means SQL Server can automatically tune its memory allocations based on the load it is processing and the demands of other Windows processes on the server.

Each page of memory used by SQL Server is assigned to one of several caches. Each cache will grow and shrink in size as required. The main caches are the Buffer Cache, which holds recently accessed database pages, and the Procedure Cache, which holds recently compiled query execution plans.

This Memory gauge shows the total amount of memory currently allocated to SQL Server.

The maximum value shown on this gauge is the lower of SQL Server’s Max Memory parameter and the amount of physical RAM on the machine. It indicates the maximum amount of memory that SQL Server can use.

SQL Server parameters can be used to influence the amount of memory that SQL Server uses. The default maximum amount of memory (Max Memory) is 2147483647 MB. The minimum memory amount allowable for max server memory is 128 MB.