Matt Sayler (sayler@thewalrus.org) on 5/15/07 wrote:
---------------------------
>I have several OS X desktop boxes with at least 2GB of memory. I've heard that
>things like Parallels and VMWare have problems with machines with that much memory
>(which makes me think that OS X magically turns on PAE at some threshold).

Physical Address Extension does exactly what it says: extends the limit on physical addresses past 32 bits. The only other reason you might want it is that it provides room in the page tables for the NX bit.

But ignoring the NX bit, the sole and exclusive reason that you need PAE is if you have more than 32 bits of physical address space. And you need physical address space only for memory or memory-mapped devices that physically exist.

If the total of your physical memory plus memory-mapped devices can fit into 4 GB, you don't need PAE. There's absolutely no reason for anything to happen at the 2GB RAM level. 3 GB of RAM should be no problem whatsoever.

PAE is the difference between the HIGHMEM4G and HIGHMEM64G options in Linux. The first adds support for RAM beyond the 1 GB kernel address space. This is "high memory" which is not permanently mapped into the kernel address space, cannot be used for kernel data structures (only pages that can be mapped into user address spaces), and is bank-switched in (with some implementation tricks to minimize SMP TLB synchronization cost) if the kernel needs to access it. The second option, which is a much smaller change, adds PAE support.

Unfortunately, the "64G" part of the name is purely theoretical. There is a (somewhat application-dependent) practical limit on the amount of high memory which can be effectively used, and memory beyond that doesn't help because you're bottlenecked on low-memory data structures. ISTR that 8G works okay, 16G has some problems, and beyond that only very specialized applications benefit. And those applications benefit a lot more from having a 64-bit address space.