RHEL / CentOS Support 4GB or more RAM ( memory )

If you have 4 GB or more RAM use the Linux kernel compiled for PAE capable machines. Your machine may not show up total 4GB ram. All you have to do is install PAE kernel package.

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 (>=4GB).

How Do I Install PAE kernel?

To install PAE kernel, use yum command:

Output:

Loading "installonlyn" plugin Setting up Install Process Setting up repositories Reading repository metadata in from local files Parsing package install arguments Resolving Dependencies --> Populating transaction set with selected packages. Please wait. ---> Downloading header for kernel-PAE to pack into transaction set. kernel-PAE-2.6.18-8.1.15. 100% |=========================| 207 kB    00:00 ---> Package kernel-PAE.i686 0:2.6.18-8.1.15.el5 set to be installed --> Running transaction check Dependencies Resolved =============================================================================  Package                 Arch       Version          Repository        Size ============================================================================= Installing:  kernel-PAE              i686       2.6.18-8.1.15.el5  updates            12 M Transaction Summary ============================================================================= Install      1 Package(s) Update       0 Package(s) Remove       0 Package(s) Total download size: 12 M Is this ok [y/N]: y Downloading Packages: (1/1): kernel-PAE-2.6.18- 100% |=========================|  12 MB    00:12 Running Transaction Test Finished Transaction Test Transaction Test Succeeded Running Transaction   Installing: kernel-PAE                   ######################### [1/1] Installed: kernel-PAE.i686 0:2.6.18-8.1.15.el5 Complete!

Just reboot the server and make sure you boot with PAE kernel i.e. 2.6.18-8.1.15.el5PAE:

Source:

http://www.cyberciti.biz/tips/redhat-enterprise-linux-4gb-plus-ram-support.html