we have CENTOS 4.7 (X86) on DELL server, I already put 32 GB RAM on it. I can use "top" or "free" to check memory and it did show 32 GB.

I check REDHAT site and found it say:

The "SMP" kernel supports a maximum of 16GB of main memory. Systems with more than 16GB of main memory use the "Hugemem" kernel.

I use "uname -r" to check kernel and it show:

# uname -r 2.6.9-78.0.13.ELsmp

It did NOT show "Hugemem", does this server kernel support 32 GB or not? If NOT how can I change kernel to "hugeme"?

Dare we ask why this isn't a 64bit server? using 32gb of memory on a processor architecture that can only directly address 4GB at a time is kind of brutal, unless this server is handling a rather large number of connection sessions at the same time. For instance, a 32bit database server will have a max of about 2.5GB of shared memory usable.

to install the hugemem kernel, yum install kernel-hugemem reboot, and test this by selecting it in the grub menu, then adjust /boot/grub.conf to suit.

read the release notes carefully on hugemem, its a very high overhead thing, as the kernel runs in a private memory space seperate from the applications, so all kernel<->user transfers are complex and slow. http://www.centos.org/docs/4/html/release-notes/as-x86/