[Leaplist] 32 vs 64 bit in servers

[Ingo Claro]

Bryan J. Smith wrote:
> Ingo Claro <miclaro at netred.cl> wrote:
>   
>> has anyone made some benchmarks and is there a significant
>> improvement?
>>     
>
> There are plenty of benchmarks for many applications.
> What is your application?
>
>   
apache with mod_php
mysql
tomcat
>> 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. ]
>
>   
I got lost here, sorry. I'm using centos 5.

>> 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.
>
>   
ok, I've got 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?
>   
I've got some with core 2 duo, others with xeon.
So, if I have 4GiB or more I should install x86_64.

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