Hi,

I have trixbox installed on a Dell Tower Server. It has 8 GB of memory and my question is if it is possible to break the 3.6 GB Memory limitation. From the google search, it says that I should be using a PAE kernel but I don't know where to start. Does it mean I have to download a new kernel or can I just recompile the kernel and enable the PAE? Is there a reference you could for me to do that? Thanks.

If you can switch to the 64bit version of Centos you will be much happier. Depending on application using the PAE kernel can cause a considerable performance hit. You can download a PAE version of your kernel (the easy way) or you can compile a new kernel(the hard way).

Edit: The PAE kernel is available from Centos via yum.

lazlow is right - don't even mess with PAE. If you really need a larger address space, just make life easy on yourself and buy a 64-bit CPU. They're pretty much commodities anyway (if you can afford 8GB of RAM, you should certainly be able to afford a 64-bit CPU and mobo).

PS:
Have you ever actually used up all your RAM and starting hitting swap space on your Dell box? If so, how? What were you doing that took up all 4GB?

PPS:
You're aware that apps are still limited to 32-bit address space unless you explicitly code them for PAE. And there are very *very* few such apps in existence ... even on Windows.

I agree that it is really worth it if you could switch to 64-bit especially with so much RAM. If you enable 32-bit emulation and things are setup right you will have no problems and you can even run 32-bit apps.

Is there a way to get away with it by getting a 64 bit kernel and recompiling it? So that I keep my setting without installing a new 64=bit distro and restore the whole configs? Thanks.

No.

To switch to 64 bit, you need to install a 64 bit distribution. You can switch to PAE by changing just the kernel. For your situation, that is the big advantage of PAE.

I'm not convinced that PAE would have any detectable performance loss vs. a non PAE 32-bit kernel. 64-bit might perform better or worse than 32-bit depending on your applications.

Some distributions have a PAE 32-bit kernel that supports 4GB user space and over 16GB physical ram. That requires PAE plus a rather ugly kludge, that is likely to have both performance and compatibility issues. If that is the only PAE kernel your distribution offers, it might not be a good idea to use it.

An ordinary PAE kernel increases physical memory support to a max of 16GB from the 3 point something that is possible without PAE. It doesn't change the limit on user space (each process still only gets 3GB). You could build that PAE kernel if it isn't the one your distribution offers.

Depending on the applications you run, you might get much better results by switching to 64-bit. Maybe enough to be worth the extra effort. If you tell us what applications you run a lot, someone might have an estimate of whether 64-bit would make a big difference. But for most applications, either we don't know or it doesn't make a difference.

Hi John,

We have installed Trixbox Distribution and this comes with CentOS distro minus the X. This is a dedicated server for VOIP and we would want a2billing enabled. Do you have idea whether it will be beneficial to switch to 64 or just use PAE instead? Thanks.

When I tried to install the kernel-PAE, i get the following:

[trixbox1.localdomain src]# yum install kernel-PAE
Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
trixbox 100% |=========================| 951 B 00:00
base 100% |=========================| 1.1 kB 00:00
trixboxaddons 100% |=========================| 951 B 00:00
updates 100% |=========================| 951 B 00:00
addons 100% |=========================| 951 B 00:00
extras 100% |=========================| 1.1 kB 00:00
Reading repository metadata in from local files
Excluding Packages from CentOS-5 - Base
Finished
Excluding Packages from CentOS-5 - Updates
Finished
Excluding Packages from CentOS-5 - Addons
Finished
Excluding Packages from CentOS-5 - Extras
Finished
Parsing package install arguments
Nothing to do
[trixbox1.localdomain src]#


Also used, the rpm but it's not installing

Can you please help? Thanks.

CentOS installs PAE by default if you have 4GB of RAM or up. Have you verified what you actually have (uname -a)?

Problem solved. I have set my grub to use the PAE kernel. My whole 8GB memory can now be detected. Thanks.

Hi,

Can anybody could help in reading the higher memory address of a 64 bit kernel.

I am having 32 bit user land on top of a 64 bit kernel. The RAM is 8G, but, facing problems in reading the higher address from the user space. HOwever, same could be easily done from a kernel space module.

Thanks in anticipation