Dell 690 only seeing 3 GB RAM (was: slow last 128MB...)

[Ben Scott]

On 4/19/07, Michael ODonnell <michael.odonnell at comcast.net> wrote:
> I'm betting it was disabling the MTRR that
> did it, not the PAE stuff...

  Dave Johnson points out that 1 GiB of your RAM exists from 4 to 5
GiB on the physical address bus.  That would imply you do need PAE.

  I've learned in other contexts that physical address space (i.e.,
hardware, which includes RAM as well as peripherals) is not the same
as virtual address space (which is what the MMU maps and presents to
each process).  Apparently this is another manifestation of that.

  I'm not sure if disabling MTRR is a good idea or not.  It may be
important for other reasons, and sometimes disabling something that is
causing a problem just leads to bigger problems down the road.
Perhaps Dave or someone else who understands it better can comment on
that aspect?

> And on one machine w/these same symptoms I've installed Debian
> running a 2.6.16 kernel.  No joy - `still sees just 3Gb...

  Well, it's not an "old kernel" problem, then.  Maybe a "kernel
config" problem.  Perhaps RH ships all their kernels with PAE enabled
now, but Debian does not?  I'm pretty sure that, back in the RHEL 3
days, RH was shipping a regular kernel and a "bigmem" kernel.

-- Ben