[K12OSN] 8 GB RAM on 32-bit Edubuntu with AMD 64 processors

Nils Breunese nils at breun.nl
Mon Nov 26 22:33:55 UTC 2007


Robert Arkiletian wrote:

> On 11/26/07, James P. Kinney III <jkinney at localnetsolutions.com>  
> wrote:
>> The PAE will be a nightmare for the heavily shared setup of LTSP. In
>> short, the system will thrash doing high page swaps to access RAM. If
>> all processes under current request are spread over multiple pages,  
>> the
>> performance will be beyond horrible.
>>
>> If you _must_ use 32-bit on a AMD-64 bit machine, put no more than  
>> 4GB
>> RAM in the system. Even then you may loose 512MB (depends on  
>> mainboard
>> chipset). I'm not sure if the Nvidia chips will loose RAM as all of  
>> my
>> servers using those have 8GB or higher and 64-bit OS.
>
> I have 4GB of ram. My kernel is using PAE so I see all 4GB. Wondering
> if performance hit (some say 6-8%)  is only for that extra 512MB or
> for the entire 4GB?
>
> Also wondering how does one disable PAE? Running 4.2.3EL the 2.6.9.x
> kernel has PAE enabled by default. Is there a kernel rpm I can
> download or will I have to recompile the kernel?

I believe there are only three different kernels available for CentOS  
4 (on which K12LTSP 4EL is based):

----------------------------------------------------------------

# yum info kernel
(...)
Summary: the linux kernel (the core of the linux operating system)

Description:
  The kernel package contains the Linux kernel (vmlinuz), the core of  
any
Linux operating system.  The kernel handles the basic functions
of the operating system:  memory allocation, process allocation, device
input and output, etc.

# yum info kernel-smp
(...)
Summary: The Linux kernel compiled for SMP machines.
Description:
  This package includes a SMP version of the Linux kernel. It is
required only on machines with two or more CPUs as well as machines with
hyperthreading technology.

Install the kernel-smp package if your machine uses two or more CPUs.

# yum info kernel-hugemem
(...)
Summary: The Linux kernel compiled for machines with 16 Gigabytes of  
memory or more.
Description:
  This package includes an SMP version of the Linux kernel which
supports systems with 16 Gigabytes of memory or more.

----------------------------------------------------------------

I don't see a PAE-kernel for CentOS 4 by the way (looking at http://mirror.centos.org/centos-4/4/os/i386/CentOS/RPMS/) 
. Are you sure you have a PAE kernel? CentOS 5 does have a package  
called kernel-PAE (looking at http://mirror.centos.org/centos-5/5/os/i386/CentOS/) 
  however, but you said you were running K12LTSP 4EL?

Info for the kernel-PAE package in CentOS 5:
----------------------------------------------------------------
This package includes a version of the Linux kernel with support for  
up to 64GB of high memory. It requires a CPU with Physical Address  
Extensions (PAE). The non-PAE kernel can only address up to 4GB of  
memory. Install the kernel-PAE package if your machine has more than  
4GB of memory.
----------------------------------------------------------------

Check which kernel packages you have installed:

# rpm -qa | grep ^kernel

Check which kernel is currently running:

# uname -a

You can also check /etc/grub.conf to see what kernel is set to boot by  
default (remember the count starts at 0).

Nils Breunese.
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