Comments on: Using more than 3 Gb of RAM with 32-bit Ubuntu 10.04 (Lucid)

By: Greg Glassman

Greg Glassman — Fri, 13 Jan 2012 20:13:22 +0000

Thanks for the write up. Worked great in our VM environment.

By: R.J. Steinert

R.J. Steinert — Wed, 15 Sep 2010 23:56:02 +0000

Perry: Thanks for the info.

By: perry

perry — Wed, 15 Sep 2010 23:26:52 +0000

Kurt: The new kernel will only become active after a reboot (and should normally be at the top of the GRUB menu, so will boot by default). Are sure you are running a 32-bit version of Ubuntu ? If you see 4 Gb before a reboot, then I’d say you don’t need the PAE kernel and are probably running a 64-bit kernel.

You can find out which kernel you are running with:

uname -a

and it’s architecture specifically with:

uname -m

If this says x86_64 or ia64, you are already running a 64-bit kernel. In this case, I can’t see any good reason to run the PAE kernel.

By: perry

perry — Wed, 15 Sep 2010 23:00:35 +0000

R.J: It seems every time a new -generic kernel update it released, the -generic-pae comes out at the same time, so from Ubuntu / Canonicals side it seems to be maintained as a first-class option with regard to security updates etc. The PAE kernel and the ‘regular’ kernel are generated from the same source tree – there isn’t a separate code base for the PAE version, just a compile time flag CONFIG_X86_PAE=y, so while different, they can’t easily diverge too much.

By: Kurt

Kurt — Wed, 15 Sep 2010 16:03:42 +0000

Thanks for this. I am kind of curious though. I never rebooted and free -m is showing the full 4gb as available. Now I am not sure what to do on reboot.

By: R.J. Steinert

R.J. Steinert — Wed, 08 Sep 2010 14:09:24 +0000

Good point, I’m now running the linux-generic-pae kernel and just ran into the ‘custom compiled kernel module’ problem with VirtualBox. I had to install the linux-headers-generic-pae package to get VirtualBox to recompile its own kernel module.

Recompiling kernel modules is a minor inconvenience, I’m more worried that the linux-generic-pae package isn’t maintained as closely as the non pae package. I’m not familiar with how Linux kernel development goes but do you think it’s possible that the stock linux Kernel that Ubuntu comes with and what comes with the linux-generic-pae package could drastically fork?

Thanks for your help!

By: perry

perry — Tue, 07 Sep 2010 23:37:08 +0000

I bit of digging ( “Enabling PAE” ), and I discovered that in fact older versions of Ubuntu didn’t have PAE enabled in the stock “linux-generic” kernel either .. you needed to use the “linux-server” kernels in 9.04 to get PAE support. With 9.10+ there are PAE kernel packages called “linux-generic-pae”. I don’t think there is any simple switch that can be flicked to make the default “linux-generic” kernel support PAE as-is – I’m assuming it hasn’t been enabled at compile time.

Personally I don’t really see installing a variant of Ubuntu’s official kernels as all that drastic – regular Ubuntu updates will typically upgrade the default “-generic” kernel several times a year anyway, and most people wouldn’t even notice that it’s happening except that they are prompted with “Restart required” after apt-get upgrades a kernel. Installing the “-pae” variant is barely any different.

[For the ultra geeky: You could download the "-generic" and equivalent "-generic-pae" kernel source and do a 'diff' between them .. I bet there are very few source code changes, other than the config flag that says "compile the PAE stuff" (no time to check that myself just now, unfortunately). ]

In the case where you are using your own custom compiled kernel modules (or custom kernel), yeh, then changing kernels is drastic – but then anyone doing that would probably be comfortable with it anyhow

By: R.J. Steinert

R.J. Steinert — Tue, 07 Sep 2010 16:16:24 +0000

Replacing the Kernel to enable PAE seems kind of drastic. Perhaps there is a way to tell Ubuntu to use the same Kernel with PAE enabled?

By: perry

perry — Tue, 07 Sep 2010 00:44:05 +0000

@Patrick: I’d say there is some other issue – the 64 bit kernel should see all your RAM without needing a PAE kernel.

By: Patrick

Patrick — Sat, 04 Sep 2010 12:03:18 +0000

Doesn’t work for me either on my ubuntu 10.04 2.6.32-24-generic-pae. I even tried installing directly the 64bits version of ubuntu 10.04 and free -m still only 3Gb. In the bios i can see the 4Gb ram installed. Computer is an HP dv6355ea.