Scaling Out Versus Scaling Up with Intel Physical Addressing Extensions (PAE)

Microsoft Windows 2000 Advanced Server and Microsoft Windows 2000 Datacenter Server have the capability to exceed the 4 GB limit with the use of Intel Physical Addressing Extensions (PAE). With these extensions, the Windows 2000 Advanced Server memory limit increases to 8 GB, and the Windows 2000 Datacenter Server memory limit increases to 32 GB.

When you use Intel PAE's, some kernel memory limitations still apply. These limitations could mean that scaling up (adding more memory) may not be as appropriate as scaling out (adding more nodes to the cluster). Three major areas of kernel memory are affected: system page table entries, non-paged pool, and paged pool. On a system with 4 GB of Random Access Memory (RAM) or less, if one of these resources is limited and tuning is not effective, it is more appropriate to scale out.

Generally, the best candidates for scaling up are applications that function mainly in user mode, such as Microsoft SQL Server 2000. These applications benefit from large memory in two circumstances:

• If they involve multiple discrete processes. For example, SQL Server 2000 can be configured for multiple instances of the service.
• If they are specifically designed to take advantage of the Address Windowing Extensions (AWE) API.

Some system services and settings can affect the amount of memory, for example:

• Terminal Services attempts to maximize system page table entries at the expense of paged pool memory.
• If you restart with the 3 GB switch, also known as "4 Gig tuning," the amount of non-paged pool is reduced to 128 MB from 256 MB.

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