CentOS - Nov 2007 - 32 Bit vs. 64 Bit Linux Server installation

Hi Centos Users

What are the advantages of 64 Bit and respective 32 Bit installation
of Centos? With PAE 32 Bit installation can address huge amount of
RAM. As a desktop system, I prefer 32 Bit installation because of
teething troubles in 64 Bit Linux world.

When would you choose 64 Bit and when 32 Bit? We are running mostly
application and web services.

Is this true that with 32 Bit architecture the kernel is limited to 1 GB RAM?

cheers
Simon
-- 
XMPP: sjolle at swissjabber.org

Simon Jolle wrote:
> Hi Centos Users
> 
> What are the advantages of 64 Bit and respective 32 Bit installation
> of Centos? With PAE 32 Bit installation can address huge amount of
> RAM. As a desktop system, I prefer 32 Bit installation because of
> teething troubles in 64 Bit Linux world.
Intel does hardware memory stuff now eh? I know that you do not get a 
performance slowdown with AMD whether it is 32-bit or 64-bit for 
anything over 1GB.
> 
> When would you choose 64 Bit and when 32 Bit? We are running mostly
> application and web services.
Amount of RAM but as you mention this new PAE thing...
> 
> Is this true that with 32 Bit architecture the kernel is limited to 1 GB
RAM?
No. It is not. The issue is not kernel support but the cpu support of 
fast memory operations for locations above 1GB which is (was?) non 
existent on Intel-base cpus and architectures.

Simon Jolle wrote:
> Hi Centos Users
>
> What are the advantages of 64 Bit and respective 32 Bit installation
> of Centos? With PAE 32 Bit installation can address huge amount of
> RAM. As a desktop system, I prefer 32 Bit installation because of
> teething troubles in 64 Bit Linux world.
>
> When would you choose 64 Bit and when 32 Bit? We are running mostly
> application and web services.
>
> Is this true that with 32 Bit architecture the kernel is limited to 1 GB
RAM?
>   
yes, max 1GB kernel space...   also, applications are limited to 3GB 
process address space each.   this isn't a problem for the likes of 
apache which spawns many seperate threads each in their own address 
space, but for a service that relies heavily on shared memory like a 
database server, being able to use more memory can be a huge win.

Also, on the x86 architecture specifically, 64bit code can often run 
significantly faster due to the larger register set (32bit x86 has only 
6 general registers, x86_64 has 14 general registers), this makes life 
much easier for compiler optimizations.


on x86 32bit, addressing over about 3-3.25GB of physical ram requires 
"PAE" which has additional CPU overhead on each page table access.  it
also requires larger page tables which eat into that limited kernel 
space.   (its 3GB rather than 4GB because the PCI/PCIX/PCI-E buses 
require significant hardware address space in the bottom 4GB range)

Simon Jolle wrote:
...
> When would you choose 64 Bit and when 32 Bit?
Some applications run faster when they are compiled
and run under x86_64. I've seen programs running 30% faster.

On i386, each process is limited to 2G RAM address space.

My own desktop is Fedora 7 x86_64 with 32bit Firefox (to use
plugins that are 32 bits only).

All our 64bit servers run x86_64 Centos 5 or RHEL 5.1.

Mogens
-- 
Mogens Kjaer, Carlsberg A/S, Computer Department
Gamle Carlsberg Vej 10, DK-2500 Valby, Denmark
Phone: +45 33 27 53 25, Fax: +45 33 27 47 08
Email: mk at crc.dk Homepage: http://www.crc.dk