Seeking mainboard recommendations (RE: Only half of RAM)

[richb-RBmg6HWzfGThzJAekONQAQ at public.gmane.org (Rich Braun)]

Richard Pieri noted:
> My first guess is that you are running a 32-bit kernel.

I haven't had any need a 32-bit kernel since about 2007.  The server-build
method that I use (PXEboot into an autoyast OpenSuSE 11.3 config) makes that
part pretty foolproof.

(Before I get into yet more details, let me summarize the symptom:

- If I put a single 4GB RAM module in the machine, it sees 4GB.
- If I put two 4GB modules into slots 1 & 3, BIOS reports 8GB but
  Linux (64-bit) sees only 4GB.
- If I move the modules to slots 1 & 2, BIOS and Linux both see
  8GB (single-channel).
- If I put 3 or 4 4GB modules into any combination of slots,
  BIOS reports correct amount but Linux always reports 8GB.

There are never any errors reported, system has been running fine without
crashing for weeks and there is nothing flaky.)

# uname -a
Linux i5lab 2.6.34.7-0.3-desktop #1 SMP PREEMPT 2010-09-20 15:27:38 +0200
x86_64 x86_64 x86_64 GNU/Linux

Looking at 'dmesg' output, I can compare this 16GB machine with another that
has 8GB.  I can't say whether the first few hundred lines of dmesg output
contains a clue (nothing obvious); when it gets to summary I see the
following:

[    0.000000] Memory: 8172504k/8912896k available (4777k kernel code, 533444k
absent, 206948k reserved, 6605k data, 892k init)

By comparison, the 8GB machine reports slightly different numbers:

[    0.000000] Memory: 8179700k/9175040k available (4775k kernel code, 788500k
absent, 206840k reserved, 6607k data, 896k init)

edwardp-mh2Nk+tgbQM at public.gmane.org noted:
> My K6-2 motherboard (DFI K6BV3+/66) requires DIMM's that are
> double-sided

That's a possibility, though the big question mark is why the BIOS splash
screen and configuration screens all report the 16GB alive & well.

David Kramer asked:
> Is it possible you're using one of those video cards that steals
> main RAM instead of having its own?

Good question.  This P55-GD65 mainboard comes with /no/ onboard graphics.  At
installation time I ventured into my basement and found a 1996-vintage PCI
adapter that works well enough to boot the system.  When I first saw this RAM
problem last month, I fished out a second adapter of a different type and the
change had no effect.

Jerry Feldman wrote:
> The one issue with Rich's system are that the memories are 2
> different speeds. What if Rich reversed the order of the memories.

Note that the symptom is basically the same with a single pair of RAM as with
two pair.  If I put one module on each side of the first dual-channel pair of
DIMM slots (1 & 3), the O/S only sees half the available RAM.  (Same whether I
put in a pair of OCZ or Corsair, and /leave out/ the second pair.)

Thanks for all the suggestions!  If any of y'all has a favorite mobo for
building mid-size servers, let me know and I'll just go out and buy a
known-good one instead of continuing to fool around with this one.  My home
budget doesn't give me what I'd really like (a Dell R710 like the ones at my
last workplace, with 18 DIMM slots...;-)  Come to think of it, it can be a bit
of a pain to configure RAM in those big servers.  Even with the Dell
cheat-sheets, it takes me 15 to 30 minutes of head-scratching to configure
those with anything more than 16GB (they can go to 72GB with 4GB DIMMs, and I
think they can take 8GB modules; the most I've done is 48GB).

-rich