What is it ?
Windows Server 2003 includes support for a startup switch that lets you tune the allocation of use of memory and memory address space. Regardless of the amount of physical memory in your system, Windows uses a virtual address space of 4 GB, with 2 GB allocated to user-mode processes (for example, applications) and 2 GB allocated to kernel-mode processes (for example, the operating system and kernel-mode drivers). On systems that have 1 GB or more of physical memory, the startup switche can be used to allocate more memory to applications (3 GB) and less memory to the operating system (1 GB). This additional virtual address space helps reduce the amount of memory fragmentation.
How beneficial is it ?
You may have read many articles on this subject before. This discussion has been going on for many years now and at times has almost reached epic proportions due to the conflicting information available from Microsoft. Long story short is that by and large, you should NOT use the /3GB switch unless you meet specific criteria, please read the following article as it demystifies the whole theory. Or read the excerpt below.
The /3GB option was intended as a short term solution to allow applications such as database servers to maintain more data in memory than a 2GB address space allowed. However, using the /3GB method to increase the user-mode memory space comes at a cost. If we have to allocate an additional 1GB of this address space to the user-mode space, then the System space is cut in half. Drivers, Heap, Paged & NonPaged Memory all have only half the resources to work with now. However, because of the way memory mapping works, cutting the kernel space in half does a lot more than just reducing the address space. Many of the structures within the kernel virtual memory space are cut back by far more than 50%.
For a process to access the full 3GB address space, the image file (application process) must have the IMAGE_FILE_LARGE_ADDRESS_AWARE flag set in the image header.
If the flag is not set in the image header, then the OS reserves the third gigabyte so that the application won’t see virtual addresses greater than 0x7FFFFFFF. You set this flag by specifying the linker flag /LARGEADDRESSAWARE when building the executable. This flag has no effect when running the application on a system with a 2-GB user address space. Therefore if you enable the /3GB switch, then applications that do not have this flag set can only use the standard 2GB of User mode memory, and the Kernel is still limited to the 1GB space – which means that 1GB of virtual memory is basically wasted !
All that is required to make it happen is a switch in the boot.ini file. The switch, /3GB, is placed
at the end of the line that executes the WinNT loading process.
[operating systems] multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows NT
Server Version 4.00" /3GB
Or you may add an additional line in your boot.ini as above to have the option to boot into either environment, with or without the switch.
What to keep in mind ?
This topic deals with the the virtual memory address space and has no relevance with the physical memory, it is however a limitation of a 32bit OS if you are running 64bit OS this not applicable. At the end of the day you must decide if your application is capable of handling this switch as an added benefit. Often times, if you are having to up the threshold of your OS handling of things and or things such as over-clocking your processor to keep up, one might worry about the logic behind it. Perhaps go for 64bit OS to begin with.