Re: ram

[lsorense@csclub.uwaterloo.ca (Lennart Sorensen)]

On Sun, Apr 09, 2006 at 04:10:26PM -0700, Peter A. H. Peterson wrote:
> Quoting Steffen Grunewald:
> > Wouldn't this mean that, in a dual CPU system, one should better use
> > 4 mem modules?
> 
> I just got an ASUS K8N-DL (because it is NUMA but fits into many ATX
> cases). It is a strange board in that CPU2 has two slots and CPU1 has
> 4 slots. The slots are also named strangely. Here's a diagram:
> 
> | | [ ] 
> | |     | | | |
> | | [ ] | | | |
> B A     | | | |
> 3 3     A A B B
>         1 2 1 2
> 
> Two banks on the left, slot B3 and slot A3. Four banks on the right,
> A1, A2, B1, and B2.
> 
> A table of says:
> 
> For CPU1		Sockets
> Channel A		A1 and A2
> Channel B		B1 and B2
> For CPU2
> Channel A		A3
> Channel B		B3
> 
> This makes you think that if you have 4GB (4x1GB) you can put two
> sticks in A3/B3 and two sticks in A1/A2.
> 
> But then you turn the page and you find this table:
> 
> "You may install 256M, 512M, 1G, 2G or 4G registered ECC DDR DIMMs
> into the DIMM sockets using the memory configurations in this
> section.
> 
> * For dual-channel configuration, the total size of memory modules
>   installed per channel must be the same for better performance.
>   Single CPU:
>    A1 + A2 = B1 + B2
>   Dual CPU:
>    A1 + A2 = B1 + B2 = A3 + B3
> * When using one DDR DIMM module, install into A1 slot only.
> * When using two DDR DIMM modules, install into A1 and A2 slots only.
> [snip]"
> 
> THe whole "A1 + A2 = B1 + B2 = A3 + B3" thing doesn't make sense
> because it makes it sound like if you want dual-channel memory on both
> CPUs you need RAM in all 6 slots, and each needs to be equal, which
> seems extremely limiting.
> 
> I put two in A1/A2 and two in A3/B3... but then that means that
> channel A has 3 GB and Channel B has 1GB.
> 
> Does anyone know what they're talking about, or has this board and has
> done some testing?
> 
> I'm baffled.

It is VERY badly written.  It seems that the way this board works is
this:

One cpu has two channels with two slots each.  The other cpu has two
channels with one slot each.  To get the best system performance you
want it to be balanced with equal amounts of ram on each channel on each
cpu.  Some ways to do this would be:

A1, A2, B1, B2 = 512M each
A3, B3 = 1GB each.
Total of 4GB ram.

Or you could do:
A1, B1, A3, B3 = 1GB ram each.
Total of 4GB ram.

It does not says that you MUST do it that way, just that you get optimal
performance that way.  You could also do:
A1, A2, A3, B1, B2, B3 = 1GB each.
Total of 6GB, but 2/3 of the ram is on cpu1 and 1/3 on cpu2.  So in that
case if you are lucky the OS will run ram hungry programs on cpu1 and
less ram hungry programs on cpu2 most of the time.  It is only one extra
cycle to go through the hypertransport link to the other cpu so it isn't
that big a deal either.

You MUST however have at least one stick of ram installed per cpu in
that cpu's memory area.  If you don't put ram in A3 or B3, then it will
not see CPU2 at all.  So for a dual cpu system you must have at least 2
sticks of ram, and would be running one stick in A1, B1, A2, or B2, and
one in A3 or B3.  You would of course loose the dual channel advantage.
It would be balanced though if the ram was the same size on both.

Asus manuals are usually very well written.  This one, at least the
section on ram, is not up to their usual standards.  Far from it.

Len Sorensen