The Linux kernel includes full PAE mode support starting with version 2.3.23, enabling access of up to 64 GB of memory on 32-bit machines. A PAE-enabled Linux-kernel requires that the CPU also support PAE.

How to update/install the PAE kernel :-

1. Please check either PAE kernel install on your server or not :-

# rpm -qa kernel\* | sort or
# rpm -qa |grep PAE or
# Uname -r

2. Chek the available PAE kernel inforamtion :-

# yum info kernel-PAE

Available Packages
Name : kernel-PAE

Summary: The Linux kernel compiled for PAE capable machines.

Description:
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.

3. Install the kernel-PAE package :-

# yum install kernel-PAE

4. Once the install has finished, take a look at which kernels are now installed on your system (rpm -qa kernel\* | sort ), confirm that the PAE kernel will be the default on system boot (less /etc/grub.config – make sure that the PAE kernel is the first in the list of kernels to be referenced), ensure that all future updates will be of the PAE kernel (edit the file /etc/sysconfig/kernel so that the bottom line now reads DEFAULTKERNEL=kernel-PAE rather than DEFAULTKERNEL=kernel) and then reboot the system.

5. just change the contents of the file /etc/sysconfig/kernel to be:

# UPDATEDEFAULT specifies if new-kernel-pkg should make
# new kernels the default
UPDATEDEFAULT=yes

# DEFAULTKERNEL specifies the default kernel package type
DEFAULTKERNEL=kernel-PAE

6. for consistency, you might as well yum install kernel-PAE-devel

7. Here is /etc/grub.conf contains :-

# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/sda5
# initrd /initrd-version.img
#boot=/dev/sda
default=1
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-53.1.14.el5PAE)
root (hd0,0)
kernel /vmlinuz-2.6.18-53.1.14.el5PAE ro root=LABEL=/
initrd /initrd-2.6.18-53.1.14.el5PAE.img
title CentOS (2.6.18-8.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-8.el5 ro root=LABEL=/
initrd /initrd-2.6.18-8.el5.img

Note :- ‘Default’ starts at 0, not 1.So you have to change “default=1 to default=0.

Once your system has rebooted, your system will be running with PAE kernel, check the kernel with uname -rmi and by using the command free -om, check that all your memory is now visible.

Enjoy Linux..!!!!