Server Memory Options

Setting the Memory Options Manually

Set min server memory and max server memory to span a range of memory values. This method is useful where system or database administrators want to configure an instance of SQL Server in conjunction with the memory requirements of other applications that run on the same computer.

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

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.

Use statistics from the Windows System Monitor to help you adjust the memory value if necessary. Change this value only when you add or remove memory, or when you change how you use your system.

Virtual Memory Manager

The 32-bit operating systems provide access to 4-gigabyte (GB) of virtual address space. 2 GB of virtual memory is private per process and available for application use. 2 GB is reserved for operating system use. All operating system editions include a switch that can provide applications with access up to 3 GB of virtual memory, limiting the operating system to 1 GB. For more information about how to use the switch memory configuration, see the Windows documentation about 4-gigabyte tuning (4GT). When the 32-bit SQL Server is running on 64-bit operating system its user available virtual address space is the full 4 GB.

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

When you are running multiple instances of the Database Engine, there are three approaches you can use to manage memory:

• 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.

Examples

The following example sets the max server memory option to 4 GB:

sp_configure 'show advanced options', 1;
GO
RECONFIGURE;
GO
sp_configure 'max server memory', 4096;
GO
RECONFIGURE;
GO

See Also

Reference

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