do I need to do something special to use more than 4 Gigs under Feisty?

[Gavin McCullagh]

Hi,

On Wed, 30 May 2007, john  wrote:

> I just installed Edubuntu 7.04 on Server running a dual opteron, dual
> core setup. 

That's a 64-Bit machine right?  Are you running 64-Bit Edubuntu?

> The mobo is a Tyan S3992G3NR. The kernel is setup as SMP and all cores
> are recognized. My problem is that only 3 Gigs of memory is recognized.
> That seems odd to me, as I would expect problems (if they occurred) to
> start at 4G. 

The 4GB limit is due to the amount of memory which can be addressed in 32
bits.

	2^32 = 4294967296

So you'd imagine you could address 4GB RAM in that much space.  However,
each PCI device gets mapped into the top end of the 32-Bit address space
and takes up some chunk of it.  This means that there's a limited amount of
address space (< 4GB) left for memory.  As a result, some amount of RAM x,
roughly

	3GB < x < 4GB 

is left addressable.  It _may_ be possible to squeeze a little more out of
it by disabling or removing PCI devices that you don't need.

> Does anyone know if there is something special one has to do to enable
> large memory support for Ubuntu Feisty?

It can be done in 32-Bit Linux, but (as I understand it) this comes at a
cost.  However, I would have thought that if you are running 64-Bit Ubuntu
Linux this shouldn't be an issue at all.  Are you?  I use "may" and
"should" because I've yet to use a 64-bit a machine with >4GB ram.

Gavin