On Fri, Jan 29, 2010 at 01:59:13PM -0800, Akemi Yagi wrote:
> 
> [from red hat bugzilla] ... Additionally, the Red
> Hat Enterprise Linux 5 PAE variant does not allow 4G of addressable memory
> per-process like the Red Hat Enterprise Linux 4 kernel-hugemem variant does.

If I remember right, this is what happened:

For a 32-bit OS, a user process can only address 4 GiBytes of memory.

But some of this memory has to be reserved by the OS itself for sundry OS needs.

Originally, Linux had it divided as 2 GiBi user + 2 GiBi system, and
it did not matter for machines with only 2 GB of physical memory
or less (right?). Then, as machines with more memory became more affordable and common,
users wanted to use this memory, so the user part was pushed up to 3+1 split,
then to 3.5+0.5 (I think).

Then Red Hat had a special patch for Linux 2.4 kernels that permitted 4+0 split,
but it was not accepted into the mainstream kernel (I guess 0 bytes
for kernel use was too little) and when the OS switched to the 2.6 kernels
with RHEL5, this custom modification disappeared.

-- 
Konstantin Olchanski
Data Acquisition Systems: The Bytes Must Flow!
Email: olchansk-at-triumf-dot-ca
Snail mail: 4004 Wesbrook Mall, TRIUMF, Vancouver, B.C., V6T 2A3, Canada