[Leaplist] 32 vs 64 bit in servers

[Bryan J. Smith]

Ingo Claro <miclaro at netred.cl> wrote:
> Hello all:
> is there really any improvement from a 32 bit to a 64 bit
> OS? appart from a memory constraint issue,

Define "memory constraint"?

There reaches many points where x86 paging will actually
regress in performance to x86-64 paging.

> has anyone made some benchmarks and is there a significant
> improvement?

There are plenty of benchmarks for many applications.
What is your application?

> I understand that with PAE you could have up to 64 GB of
> RAM,

x86 allows up to 32-bit/4GiB addressing.
x86 PAE allows up to 36-bit/64GiB** addressing.

x86_64 (AMD64/EM64T) allows up to 48-bit/256TiB addressing.
x86_64 PAE allows up to 52-bit/4PiB addressing.

NOMENCLATURE NOTE:
     x86 = i486 (i686 PAE) = IA-32
  x86-64 = AMD64 ~ IA-32e = EM64T

"Long Mode" is always x86_64 PAE (52-bit), for compatibility
with both x86 (32-bit into 48-bit) and x86 PAE (36-bit into
52-bit).  I've never seen an CPU-kernel use x86-64 without PAE.

[ **SIDE NOTE:  Just today someone pointed out that Red Hat's
x86 4G/4G memory model actually supports more than 64GiB,
and has been shown to work on AMD64 processors using
40-bit PAE.  This is a legacy throwback to the original
32-bit Athlon MP which had 40-bit EV6 addressing (long
story).  Normally the x86 3G/1G PAE kernel mode experiences
"LOMEM exhaustion" after 32GiB, which Red Hat addresses
with its x86 4G/4G memory model so up to 64GiB can be used.
It's no longer included in newer Fedora/RHEL developments
(circa release 5 for each), only x86 3G/1G PAE. ]

> so if I have under 64G, would it be better to stick with
> 32 bit,

_No_.  The general rule is any server with 4GiB or more.
There may be exceptions, but at 8GiB+, you're paging so much
that any "loss" in adding x86 libs to x86-64 libs on a server
really swings the other way.  Not sure about desktops, but
by far, servers -- run x86-64 for 4GiB.

The PAE support on x86 (i686) was _not_ designed for
performance, but a "stop-gap" solution until IA-64 "took off"
(which it never did, hence AMD x86-64 instead of IA-64).

[ NOTE:  Intel IA-64 = Itanium, IA-32e = EM64T ~ x86-64 ]

> or install a  64bit os (in the same hardware,
> for example in a C2D)

C2D?  Core 2 Duo?  Any reason you're not using a Xeon?


-- 
Bryan J Smith          Professional, Technical Annoyance
b.j.smith at ieee.org    http://www.linkedin.com/in/bjsmith
--------------------------------------------------------
I don't have a "favorite Linux distro."  I use, develop
and support community efforts, often built around Linux.
Technology and solutions are my focus, not dragging in
assumptions, marketing and other concepts which dominate
non-community developed software, which I left long ago.


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