[Beowulf] Opteron memory rank limits with DDR-400
josip at lanl.gov
Thu Jul 28 18:39:10 EDT 2005
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.
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