4GB RAM with an intel core 2 processor

Scott Long scottl at samsco.org
Thu Jan 24 10:59:42 PST 2008


Baldur Gislason wrote:
> I recently tried to upgrade my Dell workstation from 2GB to 4GB RAM.
> It's an Optiplex 755 with an Intel Core 2 Quad Q6600 processor. I'm running 7.0-RC1
> When I install the 2 extra memory modules, the kernel tells me on startup
> that it's ignoring 640MB memory above 4GB, leaving me with a total of 3.375GB
> of accessible memory. However when X starts (using a PCI-e Geforce with nvidia driver)
> the system reboots.
> Now, I took a quick look at PAE and discovered that the PAE kernel is
> not meant for anything but headless systems, most of the drivers (including USB!) are out and no
> module support either. 
> 
> Do I have any options for using my 4GB of RAM and still retain USB and the nvidia driver?
> 

At the top of the 32 bit address space is a large region that is
reserved for addressing system and peripheral devices.  It's typically
the top 256-768MB of the address space.  Any RAM that is mapped into
this space is not available, so what system designers typically do these
days is exclude this region from RAM, and map the RAM that would be
otherwise lost into the address space to be above the 32-bit limit.
That's why it says that it's "ignoring" 640MB of RAM; it knows that it's
there, but since the OS is confined to a 32-bit address space, it knows
it can't get to it.

I would guess that the reason you're getting panic/reboot in X is
because you're using a graphics card with a lot of on-board RAM.  The
video driver is trying to map this RAM into the CPU's address space, and
there simply isn't enough room for it because you have the space full
of RAM (and the above mentioned reserved space for other devices).

With the PAE kernel, modules aren't allowed because PAE caused system
data structures to change in a way that module builds couldn't cope with
at the time the PAE was written.  I believe that that might not be the
case any longer, so long as you use the "buildkernel" method and don't
try to individually compile modules.  It's worth testing.  Also, USB was
very unreliable in 64-bit environments at the time that PAE was written,
so it was also excluded.  So what's really happened here is that the PAE
kernel config file has probably just gotten stale and in need of a
review.

I don't know if PAE will fix your X video problems, though.  Probably
the better bet all around is to run amd64.

Scott




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