[CentOS] 32 Bit vs. 64 Bit Linux Server installation

John R Pierce pierce at hogranch.com
Fri Nov 9 09:12:11 UTC 2007

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)

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