IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

324 IBM TotalStorage DS6000 Series: Performance Monitoring and Tuning

5. Edit the current BOOT.INI file to include the /3GB switch as shown in Figure 9-12.

6. Restart the server for the change to take effect.

Figure 9-12 Editing the BOOT.INI to include the /3GB switch

This switch normally should be used only when a specific application recommends its use.

Typically this is where applications have been compiled to use more than 2 GB per process,

such as some components of Exchange.

9.2.9 Using PAE and AWE to access memory above 4 GB

The native 32-bit architecture of the x86 processor allows a maximum addressable memory

space of 4 GB. The Intel Physical Address Extension (PAE) is a 36-bit memory addressing

mode that allows 32-bit systems to address memory above 4 GB.

PAE requires appropriate hardware and operating system support to be implemented. Intel

introduced PAE 36-bit physical addressing with the Intel Pentium Pro processor. PAE is

supported with the Advanced and Datacenter Editions of Windows 2000 Server and the

Enterprise and Datacenter Editions of Windows Server 2003.

Windows uses 4 KB pages with PAE to map up to 64 GB of physical memory into a 32-bit (4

GB) virtual address space. The kernel effectively creates a map in the privileged mode

addressable memory space to manage the physical memory above 4 GB.

The Advanced and Datacenter Editions of Window 2000 Server and Windows Server 2003

allow for PAE through use of a /PAE switch in the BOOT.INI file. This effectively allows the

operating system to use physical memory above 4 GB.

Even with PAE enabled, the underlying architecture of the system is still based on 32-bit

linear addresses. This effectively retains the usual 2 GB of application space per user-mode

process and the 2 GB of kernel mode space, because only 4 GB of addresses are available.

However, multiple processes can immediately benefit from the increased amount of

Important: The /3GB switch actually works for all versions of Windows 2000 Server and

Windows Server 2003. However, you should use it only when running Advanced Edition or

Datacenter Edition.

Standard Edition can allocate to user-mode applications at most 2 GB. If the /3GB switch is

configured in the BOOT.INI file, then the privileged-mode kernel is restricted to 1 GB of

addressable memory without the corresponding increase for user-mode applications. This

effectively means 1 GB of address space is lost.

Write /3GB switch to enable 3GB of memory

for user-mode applications.