PAE가 먼지.. 잘 정리된 걸로 보입니다.

http://www.held.org.il/blog/?p=79

PAE (Physical Address Extension) is a “workaround” for letting x86-32bit(!) OS see more than 4GB of RAM. 4GB is the limit for 32bit memory addresses. PAE is not needed and not implemented on x86-64 processors when 64 bit (”long mode“) is enabled.

In short, it simply adds 4 bits to the memory addresses (32bit -> 36bit) and one more level of memory-lookup-hierarchy, and: voila, OS can access up to 64GB ram (which is not “science fiction” these crazy days..). Of course, a single 32bit process is not aware, and would still only have a 4GB of virtual address space, even with PAE.

PAE is hardware supported since Intel Pentium Pro (back in the mid 90s..).

On 32bit Windows, PAE used to be disabled by default, but since WinXP SP2 windows is PAE-enabled by default. I’ve also seen some Linux distros enabling PAE in their default kernel recently.

1. Main reason for PAE being disabled by default, seems to be hardware compatibility: hardware with no PAE support can’t boot a PAE-enabled kernel. That’s mostly history now, anyway, for all recent x86 processors support PAE.
2. Performance penalty is very low (according to RedHat average is 1% and no more than 10%). Of course it depends on your exact scenarios, etc etc.
3. As a friend suggested me: in most cases PAE is not needed, for x86-64 is so widely spread. One can simply run his 32bit apps on a 64bit OS. PAE lets the OS see up to 64GB, while x86-64 (current implementations) lets the OS see at least 16TB! So my main conclusion is.. that PAE is dead. All modern x86 processors (since ~year 2006) have x86-64 support.