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By default in 32 bit system, 2 GB is allocated to user space programs and 2GB for kernel space programs.

Now, if we add /3GB option in boot.ini user and kernel space get respectively 3 and 1GB.

We are using one server which is running MS Windows 2003 32bit with /3GB option, /PAE option with 4GB RAM and 4GB page file, how these memory are divided in user and kernel space processes?

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  • What OS are you talking about? I assume some variant of Windows? Please edit your question and add this info. – Sven Jan 02 '13 at 15:17
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    PAE is a physical memory setting. It has nothing to do with virtual memory. (The 2GB allocated to user space programs and the 2GB for kernel space is virtual memory. It applies even if you only have 1GB of physical memory.) – David Schwartz Jan 02 '13 at 22:30
  • David, thank you! That is the single most common error new Windows admins make when learning about virtual memory. Heck, half of them don't even understand the difference between physical and virtual memory. All Windows admins should be required to read Mark Russinovich's blog series on Windows memory management. http://blogs.technet.com/b/markrussinovich/archive/2008/07/21/3092070.aspx – John Homer Jan 03 '13 at 17:38

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/PAE doesn't change the user and kernel virtual address space assignments that were changed by using /3GB, but, you should not use /3GB and /PAE at the same time. Per Microsoft:

When the physical RAM in the system exceeds 16 GB and the /3GB switch is used, the operating system will ignore the additional RAM until the /3GB switch is removed. This is because of the increased size of the kernel required to support more Page Table Entries.

You need as much kernel memory space as possible (more than 1GB) just for the page table entries to describe more than 16GB of memory.

Edit: Also per Microsoft:

The conflict is when we use the /3GB with /PAE simultaneously. When we are using PAE the OS needs to manage lot of resources which needs more memory than 1 GB. So certain operations have performance issue and then fail intermittently.

Ryan Ries
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  • I once made the mistake of enabling /3GB and /PAE at the same time on a system with > 16 GB. That *definitely* teached me to never do it again. – Massimo Jan 03 '13 at 07:31