Recipe 3.15 Enabling the /3GB Switch to Increase the LSASS Cache

3.15.1 Problem

You are using more than 1 GB of memory on your domain controllers and want to enable the /3GB switch so that the LSASS process can use more memory.

3.15.2 Solution

Edit the boot.ini file on the domain controller to contain the /3GB switch:

[boot loader]
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINDOWS="Windows Server 2003" /3GB

Restart the computer.

On Windows Server 2003, you can edit the boot.ini file by opening the System applet in the Control Panel. Click the Startup and Recovery tab and click the Edit button.

On Windows 2000, it is not so easy. You need to open an Explorer window, select Tools Folder Options, and click the view tab. Uncheck "Hide protected operating system files (Recommended)," and check "Show hidden files and folders." Now browse to the root of your operating system partition (e.g., C:) and edit the boot.ini file with a text editor.

3.15.3 Discussion

When computers are referred to as 32 or 64-bit computers that means they support memory addresses that are 32 or 64 bits long. This is the total available memory (virtual and real) that can be processed by the system. Since the days of Windows NT, Microsoft has split memory allocation in half by giving applications up to 2 GB and the Windows kernel 2 GB of memory to use (32 bits of address space = 2^32 = 4 GB). In many cases, administrators would rather allocate more memory to applications than to the kernel. For this reason, Microsoft developed the /3GB switch to allow applications to use up to 3 GB of memory, leaving the kernel with 1 GB.

The /3GB switch is supported only on Windows 2000 Advanced Server, Windows 2000 Datacenter Server, Windows Server 2003 Enterprise Edition, and Windows Server 2003 Data Center Edition, and should be used only if the computer has more than 1 GB of physical memory. For a good description of how LSASS uses memory, see MS KB 308356.

3.15.4 See Also

MS KB 99743 (Purpose of the BOOT.INI File in Windows 2000 or Windows NT), MS KB 291988 (A Description of the 4 GB RAM Tuning Feature and the Physical Address Extension Switch), and MS KB 308356 (Memory Usage By the Lsass.exe Process on Windows 2000-Based Domain Controllers)

    Chapter 3. Domain Controllers, Global Catalogs, and FSMOs
    Chapter 6. Users
    Appendix A. Tool List