[root@web02 log]# dmesg | grep 4GB
Warning only 4GB will be used.
[root@web02 log]# yum install kernel-PAE
Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
extras                    100% |=========================| 1.1 kB    00:00    
updates                   100% |=========================|  951 B    00:00    
base                      100% |=========================| 1.1 kB    00:00    
addons                    100% |=========================|  951 B    00:00    
Reading repository metadata in from local files
primary.xml.gz            100% |=========================| 248 kB    00:02    
################################################## 481/481
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-2.6.18-53.1.19 100% |=========================| 261 kB    00:02    
---> Package kernel-PAE.i686 0:2.6.18-53.1.19.el5 set to be installed
--> Running transaction check

Dependencies Resolved

=============================================================================
 Package                 Arch       Version          Repository        Size
=============================================================================
Installing:
 kernel-PAE              i686       2.6.18-53.1.19.el5  updates            13 M

Transaction Summary
=============================================================================
Install      1 Package(s)        
Update       0 Package(s)        
Remove       0 Package(s)        

Total download size: 13 M
Is this ok [y/N]: y
Downloading Packages:
(1/1): kernel-PAE-2.6.18- 100% |=========================|  13 MB    00:52    
warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID e8562897
Importing GPG key 0xE8562897 "CentOS-5 Key (CentOS 5 Official Signing Key) <centos-5-key@centos.org>" from http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
Is this ok [y/N]: y
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
  Installing: kernel-PAE                   ######################### [1/1]

Installed: kernel-PAE.i686 0:2.6.18-53.1.19.el5
Complete!

[root@web02 log]# yum intall kernel-PAE-devel

[root@web02 log]# cd /boot/grub

[root@web02 log]# vi grub.conf

#boot=/dev/cciss/c0d0
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-53.1.19.el5PAE)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-53.1.19.el5PAE ro root=LABEL=/ rhgb quiet
        initrd /initrd-2.6.18-53.1.19.el5PAE.img
title CentOS (2.6.18-53.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-53.el5 ro root=LABEL=/ rhgb quiet
        initrd /initrd-2.6.18-53.el5.img

default=1 이 되어 있는 것을 0 으로 변경 부팅 커널 변경

wq!

[root@web02 log]# sync;sync;sync;reboot

위의 grub.conf 파일 순서를 변경 하지 않으면 4GB 이상 메모리로 올라오지 않는다.