[Beowulf] Opteron memory rank limits with DDR-400

[Josip Loncaric]

Vincent Diepeveen wrote:
> Quad opteron dual core 1.8Ghz


It sounds like you've got 16 single-rank memory modules, 4/Opteron, so 
you would not see the issue I'm concerned about.

Opteron memory controller apparently needs either downclocking to 
DDR333, or a recent Opteron revision plus an extra cycle of memory 
access latency, when driving 8 ranks of DDR400 memory.  Downclocking is 
automatically enforced by some BIOSes.

See the discussion at http://forums.amd.com/index.php?showtopic=49274 
for more detail...  There is much confusion on this topic.  The most 
interesting comment therein is:

> Of course Opterons can handle more than 4 banks, but then only at DDR333.
> 
> Exceptional to this are the new E Revision CPUs, which are able to
> handle more than 4 banks with DDR400. That is you can run DDR400 timings
> with 4 dual rank modules, but it will add another cycle to the memory
> access. This is referred as "Command Rate" in most bioses and could be
> 1T or 2T. In the case of 4 dual rank modules, DDR400 (and an E-Rev. CPU)
> it must be set at 2 T. So you get more bandwidth, but worse access
> timings compared to DDR333 and 1T.

Sincerely,
Josip