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:
> We have a number of Dell Precision 690 boxes with 4 1Gb DIMMs
> installed but our RHAT WS3 kernel ...

  RHEL 3 is pretty old.  I think it might be old enough that you need
to install a separate, special kernel package for systems which "large
memories".

  If you can, you might want to grab a RHEL 5 eval CD image from the
Red Hat website (or just grab a non-eval CentOS 5) and do a test
install, to see if the newer code sees all the RAM.

> We have a number of these machines so I claim that it's not just
> one system or just one DIMM.

  Seems reasonable.

> ...so it seems like right around the 3Gb mark the BIOS has
> marked the next 1Gb as "uncachable" or "reserved" ...

  Some background info, FYI: Remember that on the i386 platform, we
have a 32-bit (4 GiB) virtual address space.  System hardware has to
map its ROM and I/O into that 32-bit space.  From what I understand,
this is usually done starting right around the 3 GiB mark.  This was
never a problem until fairly recently, since nobody had that much RAM
anyway.

  Sound familiar?  Yup, that's right, just like the 640 KiB limit that
the 8086 architecture was always hitting.  Same problem all over
again.  I'm told that Windows XP even limits physical RAM usage to
less than 3 GiB for this reason: A lot of the drivers assume virtual
address space above 3 GiB is untouchable.  Twenty years later, and
Microsoft is still making the same mistakes they did in MS-DOS 2.0.

> FWIW, memtest86+ reports (and is apparently happy with) all 4Gb.

  Since memtest86 doesn't use any of the peripheral hardware (other
than the text-mode screen), I'm guessing it can do things like move or
turn off addresses used by hardware, which would make sense.

> I'd be much obliged for any illumination ...

  If you're running Linux on Dell, I highly recommend the
linux-poweredge mailing list hosted by Dell.  See
<http://linux.dell.com>.  It's nominally about PowerEdge servers, but
a lot of the discussion is either generic and also applies to, or is
even explicitly about, the Precision line.  There are some real smart
cookies on that list, including Dell's senior Linux engineers (the
guys actually writing the drivers).

-- Ben