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What is PAE?

Physical Address Extension is a technology which allows 32 bit operating systems to use up to 64 Gb of memory (RAM), something which is normally achieved by switching to a 64 bit system. PAE is supported on the majority of computers today and it is an easy procedure to enable it in Ubuntu, if it is not already. To check if your processor supports PAE, try

grep --color=always -i PAE /proc/cpuinfo

In general, a proper 64 bit system is recommended if you have 4 Gb or more memory and/or want to get the full benefit of the 64 bit architecture (see 32bit_and_64bit), but in some cases making a complete switch to 64 bit is not desirable, and using PAE can be a viable compromise.

It should be noted that there is no Virtual Address Extension. i.e. The applications still get the same amount of virtual memory since the Virtual addresses are still 32 bits while the Physical Addresses are of 36 bits. Therefore PAE is not at all equivalent to a 64 bit system.

How to Enable PAE

Ubuntu 10.04 LTS (Lucid Lynx)

Both the CD and DVD installer of Ubuntu 10.04 automatically installs the PAE enabled kernel if it detects more than 3 Gb of available memory. In the case of the liveCD, a working network connection is required, since the PAE enabled kernel packages are not present on the CD.

If you need to enable PAE manually, follow the instructions for Karmic below.

Ubuntu 9.10 (Karmic Koala)

The PAE enabled kernel can be installed using the Synaptic Package Manager (accessible from the System menu under Administration -> Synaptic Package Manager): The relevant packages are called "linux-generic-pae" and "linux-headers-generic-pae" and should be easily found with a search for "pae".

Alternatively they can be installed using either apt-get or aptitude through the terminal:

sudo aptitude install linux-generic-pae linux-headers-generic-pae

After a reboot the PAE kernel should be booted as the default option in the GRUB boot menu.

To confirm that PAE was enabled correctly you can use the system monitor form the System menu under Administration -> System Monitor. The first tab shows usable memory.

DVD installer

If installing from the DVD version of Ubuntu 9.10 (both live and alternate), it will automatically install the PAE enabled kernel if it detects more than 3 Gb of available memory, it should work straight out of the box.

Removing non-PAE kernels

If PAE was successfully enabled you may want to remove the meta-packages for the normal kernel, to disable unecessary updates to the non-PAE kernels

sudo aptitude remove linux-generic linux-image-generic linux-headers-generic

If you want to remove all of the old kernels from your boot menu, it seems like you have to do it all manually:

sudo aptitude remove linux-image-<version number>-generic linux-headers-<version number>-generic

Something went wrong

If something went wrong, just remove the PAE kernel packages using either synaptic or apt-get/aptitude. If you did not specifically remove the non-PAE kernel packages, they should still be present on the system and automatically selected as the default when the PAE kernel is removed.

Ubuntu 9.10 and later

The relevant packages are:

linux-image-generic-pae  linux-image-<version number>-generic-pae linux-headers-generic-pae linux-headers-<version number>-generic-pae

CategoryKernel

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