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

[john]

Thanks Gavin and Simon for the quick responses. I'll fill in a bit here:

When I go to /boot and do:

cat config-2.6.20-16-generic | grep CONFIG_HIGHMEM

I get:

CONFIG_HIGHMEM4G=y
# CONFIG_HIGHMEM64G is not set
CONFIG_HIGHMEM=y

So I think I may need to recompile the kernel for memory higher than 4G.

As you point out these are 64 bit CPU's but I have hesitated to run
the 64 bit version of Ubuntu because it still seems to have problems
with flash etc (or possibly flash has problems with it ;-) ), which
I'll need to be successful with this project. Also I was concerned
that Feisty AMD64 was less mature than the 32 bit version of x86
Ubuntu. I assumed the high memory issue had been solved a long time
back so I didn't think I would run in to it here.

My hope is that this box will be able to scale out to serve 100 or so
LTSP clients across our LAN, hence the need for lots of memory and
stuff like flash.

Any thoughts?

John





On 5/30/07, Gavin McCullagh <gmccullagh at gmail.com> wrote:
> 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
>
>
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