The /3GB switch changes the way that the Microsoft Windows 4 GB virtual address space is split up. Instead of splitting the 4 GB virtual address space into 2 GB of user mode virtual address space and 2 GB of kernel mode virtual address space, the split is modified to 3 GB of user mode virtual address space and 1 GB of kernel mode virtual address space. In certain scenarios, this split can starve the kernel for memory and cause system-wide problems. Additionally, applications must be specifically compiled to take advantage of this larger address space.
The ability to use the /3GB switch in Windows SharePoint Services was eliminated when large file support was introduced in Windows SharePoint Services 2.0 Service Pack 2 (SP2). Large file support requires using large kernel mode memory buffers that cause instability in Microsoft Internet Information Services (IIS) 6.0 when the /3GB switch is used.
Reducing the size of the kernel mode memory buffers when in /3GB mode causes unacceptable performance in Windows SharePoint Services file-transfer operations. Therefore, a decision was made not to support the /3GB switch in Windows SharePoint Services. Windows SharePoint Services will run if you use the /3GB switch. However, the operating system will become unstable in scenarios in which the server is under stress.
The appropriate solution to use in an environment in which Windows SharePoint Services requires a large volume of memory is a 64-bit version of Windows SharePoint Services on a 64-bit operating system. Microsoft Windows SharePoint Services 3.0 and Microsoft Office SharePoint Server 2007 support 64-bit environments.