Use the two server memory options, min server memory and max server memory, to reconfigure the amount of memory (in megabytes) in the buffer pool used by an instance of Microsoft SQL Server.
By default, SQL Server can change its memory requirements dynamically based on available system resources. The default setting for min server memory is 0, and the default setting for max server memory is 2147483647. The minimum amount of memory you can specify for max server memory is 16 megabytes (MB).
When SQL Server is using memory dynamically, it queries the system periodically to determine the amount of free physical memory. SQL Server uses the memory notification API QueryMemoryResourceNotification to determine when the buffer pool may allocate memory and release memory.
Allowing SQL Server to use memory dynamically is recommended; however, you can set the memory options manually and restrict the amount of memory that SQL Server can access. Before you set the amount of memory for SQL Server, determine the appropriate memory setting by subtracting, from the total physical memory, the memory required for the operating system, and any other instances of SQL Server (and other system uses, if the computer is not wholly dedicated to SQL Server). This difference is the maximum amount of memory you can assign to SQL Server.
Setting the Memory Options Manually
Use min server memory to guarantee a minimum amount of memory available to the buffer pool of an instance of SQL Server. SQL Server will not immediately allocate the amount of memory specified in min server memory on startup. However, after memory usage has reached this value due to client load, SQL Server cannot free memory from the allocated buffer pool unless the value of min server memory is reduced.
Use max server memory to prevent the SQL Server buffer pool from using more than the specified amount of memory, thus leaving remaining memory available to start other applications quickly. SQL Server does not immediately allocate the memory specified in max server memory on startup. Memory usage is increased as needed by SQL Server until reaching the value specified in max server memory. SQL Server cannot exceed this memory usage unless the value of max server memory is raised.
Before reducing the max server memory value, use Performance Monitor to examine the SQLServer:Buffer Manager performance object while under a load, and note the current values of the Stolen pages and Reserved pages counters. These counters report memory as the number of 8K pages. max server memory should be set above the sum of these two values to avoid out-of-memory errors. An approximate value for the lowest reasonable max server memory setting (in MB) is ([Stolen pages] + [Reserved pages])/ 100. To reduce the max server memory you may need to restart SQL Server to release the memory. For information about how to set memory options, see How to: Set a Fixed Amount of Memory (SQL Server Management Studio).
Maximize Data Throughput for Network Applications
To optimize system memory use for SQL Server, you should limit the amount of memory that is used by the system for file caching. To limit the file system cache, make sure that Maximize data throughput for file sharing is not selected. You can specify the smallest file system cache by selecting Minimize memory used or Balance.
To check the current setting on your operating system
- Click Start, then click Control Panel, double-click Network Connections, and then double-click Local Area Connection.
- On the General tab, click Properties, select File and Printer Sharing Microsoft Networks, and then click Properties.
- If Maximize data throughput for network applications is selected, choose any other option, click OK, and then close the rest of the dialog boxes.
AWE Memory Under Windows Server 2003
Under Windows Server 2003, SQL Server can use Address Windowing Extensions (AWE) memory to further assist in load balancing its own memory requirements with those of the operating system. This balancing between SQL Server and the operating system is subject to the constraints of the min server memory and max server memory options. If the server hardware supports Hot-Add Memory, additional physical memory can be added to the server as necessary without requiring a restart. For more information about the awe enabled configuration option, see awe enabled Option. For more information, see Hot Add Memory.
Ideally, allocate as much memory as possible to SQL Server without causing the system to swap pages to disk. The threshold varies depending on your system. For example, on a 32-gigabyte (GB) system exclusively dedicated to SQL Server, 30-31 GB might be an appropriate maximum threshold for SQL Server; on a 64-GB system, 60-62 GB might be an appropriate threshold.
Virtual Memory Manager
The 4-GB address space is mapped to the available physical memory by the Windows Virtual Memory Manager (VMM). The physical memory accessible by AWE therefore depends on which operating system you are using. For more information on the amount of physical memory supported by different operating systems, see the Windows documentation “Memory Limits for Windows Releases”.
Virtual address systems allow the over-commitment of physical memory, so that the ratio of virtual to physical memory can exceed 1:1. As a result, larger programs can run on computers with a variety of physical memory configurations. However, using significantly more virtual memory than the combined average working sets of all the processes can cause poor performance.
The min server memory and max server memory options are advanced options. If you are using the sp_configure system stored procedure to change these settings, you can change them only when show advanced options is set to 1. These settings take effect immediately without a server restart.
Running Multiple Instances of SQL Server
- Use max server memory to control memory usage. Establish maximum settings for each instance, being careful that the total allowance is not more than the total physical memory on your machine. You might want to give each instance memory proportional to its expected workload or database size. This approach has the advantage that when new processes or instances start up, free memory will be available to them immediately. The drawback is that if you are not running all of the instances, none of the running instances will be able to utilize the remaining free memory.
- Use min server memory to control memory usage. Establish minimum settings for each instance, so that the sum of these minimums is 1-2 GB less than the total physical memory on your machine. Again, you may establish these minimums proportionately to the expected load of that instance. This approach has the advantage that if not all instances are running at the same time, the ones that are running can use the remaining free memory. This approach is also useful when there is another memory-intensive process on the computer, since it would insure that SQL Server would at least get a reasonable amount of memory. The drawback is that when a new instance (or any other process) starts, it may take some time for the running instances to release memory, especially if they must write modified pages back to their databases to do so. You may also need to increase the size of your paging file significantly.
- Do nothing (not recommended). The first instances presented with a workload will tend to allocate all of memory. Idle instances or instances started later may end up running with only a minimal amount of memory available. SQL Server makes no attempt to balance memory usage across instances. All instances will, however, respond to Windows Memory Notification signals to adjust the size of their buffer pools. Windows does not balance memory across applications with the Memory Notification API. It merely provides global feedback as to the availability of memory on the system.
You can change these settings without restarting the instances, so you can easily experiment to find the best settings for your usage pattern.
Enable the Lock Pages in Memory Option (Windows)
The Windows policy Lock Pages in Memory option is disabled by default. This privilege must be enabled to configure Address Windowing Extensions (AWE). This policy determines which accounts can use a process to keep data in physical memory, preventing the system from paging the data to virtual memory on disk. On 32-bit operating systems, setting this privilege when not using AWE can significantly impair system performance. Locking pages in memory is not required on 64-bit operating systems.
Use the Windows Group Policy tool (gpedit.msc) to enable this policy for the account used by SQL Server. You must be a system administrator to change this policy.
For a table that lists the maximum server memory values.
To enable the lock pages in memory option
- On the Start menu, click Run. In the Open box, type gpedit.msc.The Group Policy dialog box opens.
- On the Group Policy console, expand Computer Configuration, and then expand Windows Settings.
- Expand Security Settings, and then expand Local Policies.
- Select the User Rights Assignment folder.The policies will be displayed in the details pane.
- In the pane, double-click Lock pages in memory.
- In the Local Security Policy Setting dialog box, click Add.
- In the Select Users or Groups dialog box, add an account with privileges to run sqlservr.exe.