Monitor Memory Usage

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Monitor an instance of SQL Server periodically to confirm that memory usage is within typical ranges.

To monitor for a low-memory condition, use the following object counters:

  • Memory: Available Bytes

  • Memory: Pages/sec

The Available Bytes counter indicates how many bytes of memory are currently available for use by processes. The Pages/sec counter indicates the number of pages that either were retrieved from disk due to hard page faults or written to disk to free space in the working set due to page faults.

Low values for the Available Bytes counter can indicate that there is an overall shortage of memory on the computer or that an application is not releasing memory. A high rate for the Pages/sec counter could indicate excessive paging. Monitor the Memory: Page Faults/sec counter to make sure that the disk activity is not caused by paging.

A low rate of paging (and hence page faults) is typical, even if the computer has plenty of available memory. The Microsoft Windows Virtual Memory Manager (VMM) takes pages from SQL Server and other processes as it trims the working-set sizes of those processes. This VMM activity tends to cause page faults. To determine whether SQL Server or another process is the cause of excessive paging, monitor the Process: Page Faults/sec counter for the SQL Server process instance.

For more information about resolving excessive paging, see the Windows operating system documentation.

Isolating Memory Used by SQL Server

By default, SQL Server changes its memory requirements dynamically, on the basis of available system resources. If SQL Server needs more memory, it queries the operating system to determine whether free physical memory is available and uses the available memory. If SQL Server does not need the memory currently allocated to it, it releases the memory to the operating system. However, you can override the option to dynamically use memory by using the min server memory, and max server memory server configuration options. For more information, see Server Memory Options.

To monitor the amount of memory that SQL Server uses, examine the following performance counters:

  • Process: Working Set

  • SQL Server: Buffer Manager: Buffer Cache Hit Ratio

  • SQL Server: Buffer Manager: Database Pages

  • SQL Server: Memory Manager: Total Server Memory (KB)

The Working Set counter shows the amount of memory that is used by a process. If this number is consistently below the amount of memory that is set by the min server memory and max server memory server options, SQL Server is configured to use too much memory.

The Buffer Cache Hit Ratio counter is specific to an application. However, a rate of 90 percent or higher is desirable. Add more memory until the value is consistently greater than 90 percent. A value greater than 90 percent indicates that more than 90 percent of all requests for data were satisfied from the data cache.

If the Total Server Memory (KB) counter is consistently high compared to the amount of physical memory in the computer, it may indicate that more memory is required.

Determining Current Memory Allocation

The following query returns information about currently allocated memory.

SELECT
(physical_memory_in_use_kb/1024) AS Memory_usedby_Sqlserver_MB,
(locked_page_allocations_kb/1024) AS Locked_pages_used_Sqlserver_MB,
(total_virtual_address_space_kb/1024) AS Total_VAS_in_MB,
process_physical_memory_low,
process_virtual_memory_low
FROM sys.dm_os_process_memory;