PAE kernel and 4GB of memory

[Bill Davidsen]

Howard Wilkinson wrote:
> I am looking for a definitive answer to the question of where the PAE 
> kernels become useful. I have seen various articles that mention needing 
> PAE kernels if you have more then 4GB of physical memory in a 32-bit 
> processor environment. I have also seen statements that say you need 
> them if you have 4GB or more of memory. Now which is right? Also, even 
> if you need a PAE kernel because the last few bytes are not addressable 
> when you have exactly 4GB is this useful or is the trade off of larger 
> page tables and pages going to eat any benefit of being able to address 
> these few bytes and if so when does the PAE kernel become useful?
> 
The answer is "it depends." If you have 4GB or more of usable RAM PAE 
will let you address all of it, and may increase you process max size 
depending on the options in your non-PAE kernel.

Look at /proc/mtrr to see what memory the OS thinks you have. If it 
doesn't agree with reality, use dmesg to look at the e820 registers, 
your BIOS may not set them correctly, the sizes should add up to 
physical memory, give or take memmapped hardware. If you have memort in 
mtrr ABOVE 4GB, PAE should help you.