Recommendations...

[Jerry Feldman]

On 06/15/2010 01:48 PM, Jon 'maddog' Hall wrote:
> It is true that a 32-bit machine can only access 4GB, and sometimes even
> less than that (depending on how the application address space is
> organized) in one *virtual* address space, but this does not necessarily
> stop the kernel from "using all of RAM".  It is just that various parts
> of multiple virtual address spaces get mapped into the physical memory
> of the machine.  It was this concept that allowed the old PDP-11s, which
> had only a 64K memory address space (128K with separate "instruction"
> and "data" address spaces) utilize all of the physical memory on
> machines that had multiple megabytes of RAM.
>
> So even a "32-bit" OS could fully utilize the real memory of a 64-bit
> CPU machine having multiple gigabytes of RAM if its memory management
> software allows....it is just that the applications are limited to a
> 32-bit space at one time.
>   
The Linux 32-bit kernel supports PAE (the extension that allows access
to more than 3GB RAM).  The other issue with 32-bit is with 32-but
applications as they are also limited in virtual space.  One of the
things I tested a few years ago was performance. Some applications and
benchmarks ran faster in 32-bit mode than in 64-bit mode, but some will
run faster in 64-bit mode. My testing was on both 32-bit and 64-bit
Linux on X86/X86_64 as well as Linux on IA64. The X86_64 benchmarks beat
the IA64 in many cases.

Some technical advantages of a 64-bit kernel is that the X86_64 chips
use linear addressing in 64-bit mode where 32-bit is segmented. There
are some other chip related advantages that make a 64-bit Linux OS
perform better than the same OS in 32-bit mode. Graphics performance is
also better in 64-bit mode.

-- 
Jerry Feldman <gaf at blu.org>
Boston Linux and Unix
PGP key id: 537C5846
PGP Key fingerprint: 3D1B 8377 A3C0 A5F2 ECBB  CA3B 4607 4319 537C 5846


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